resolve: Improve import failure detection and lay groundwork for RFC 1422

This PR improves import failure detection and lays some groundwork for RFC 1422.
More specifically, it
 - Avoids recomputing the resolution of an import directive's module path.
 - Refactors code in `resolve_imports` that does not scale to the arbitrarily many levels of visibility that will be required by RFC 1422.
  - Replaces `ModuleS`'s fields `public_glob_count`, `private_glob_count`, and `resolved_globs` with a list of glob import directives `globs`.
  - Replaces `NameResolution`'s fields `pub_outstanding_references` and `outstanding_references` with a field `single_imports` of a newly defined type `SingleImports`.
 - Improves import failure detection by detecting cycles that include single imports (currently, only cycles of globs are detected). This fixes #32119.

r? @nikomatsakis

Only allow using the atomic intrinsics on integer types

Using these with non-integer types results in LLVM asserts. Atomic operations on non-integer types will require values be transmuted into an integer type of suitable size.

This doesn't affect the standard library since `AtomicBool` and `AtomicPtr` currently use `usize` for atomic operations.

r? @EddyB

process cycles as soon as they are detected

We used to wait for the recursion limit, but that might well be too
long!

Fixes #32326
r? @aturon

When deciding on a coinductive match, we were examining the new
obligation and the backtrace, but not the *current* obligation that goes
in between the two.  Refactoring the code to just have the cycle given
as input also made things a lot simpler.

We used to wait for the recursion limit, but that might well be too
long!

check constants even if they are not used in the current crate

For now this is just a `warn`-by-default lint. I suggest to make it a `deny`-by-default lint in the next release cycle (so no dependencies break), and then in another release cycle move to an error.

cc #19265
cc #3170

add missing spaces in #16758 warning

I should probably add a test.

Fix a couple of dead links on core::num::ParseFloatError docs

rBreak Critical Edges and other MIR work

This PR is built on top of #32080.

This adds the basic depth-first traversals for MIR, preorder, postorder and reverse postorder. The MIR blocks are now translated using reverse postorder. There is also a transform for breaking critical edges, which includes the edges from `invoke`d calls (`Drop` and `Call`), to account for the fact that we can't add code after an `invoke`. It also stops generating the intermediate block (since the transform essentially does it if necessary already).

The kinds of cases this deals with are difficult to produce, so the test is the one I managed to get. However, it seems to bootstrap with `-Z orbit`, which it didn't before my changes.

mk: Add configure option for disabling codegen tests

Our `codegen` test suite requires the LLVM `FileCheck` utility but unfortunately
this isn't always available in all custom LLVM roots (e.g. those specified via
`--llvm-root`). This commit adds a `./configure` option called
`--disable-codegen-tests` which will manually disable running these tests. In
the case that this option is passed we can forgo the need for the `FileCheck`
executable. Note that we still require `FileCheck` by default as we will attempt
to run these tests.

Closes #28667

Our `codegen` test suite requires the LLVM `FileCheck` utility but unfortunately
this isn't always available in all custom LLVM roots (e.g. those specified via
`--llvm-root`). This commit adds a `./configure` option called
`--disable-codegen-tests` which will manually disable running these tests. In
the case that this option is passed we can forgo the need for the `FileCheck`
executable. Note that we still require `FileCheck` by default as we will attempt
to run these tests.

Closes #28667

Fix typo in TxCtxt::crate_disambiguator() and add test case.

r? @nikomatsakis

Fixes #32554

Also adds a FromIterator impl for BitVector to allow construction of a
BitVector from an iterator yeilding bools.

Implement `values_mut` on `{BTree, Hash}Map`

https://github.com/rust-lang/rust/issues/32551

Stop accepting declarations when looking for definitions

eternally_available linkage isn't legal for declarations anyway, so the
check for an externally_available declaration should never succeed, so
let's remove it.

rustbuild: Fix compile on OSX for 10.7

This commit should help configure our OSX rustbuild builder for targeting 10.7.
A key part of this is using `libc++` instead of `libstdc++` as apparently it's
more filled out and otherwise LLVM's cmake configuration would fail.

librustc: Add bug!(), bug_span!() macros as unified entry points for internal compiler errors

The macros pass `file!()`, `line!()` and `format_args!(...)` on to a cold, never-inlined function, ultimately calling `session::{span_,}bug_fmt` via the tcx in tls or, failing that, panicking directly.

cc @EddyB
r? @nikomatsakis

allow RUST_BACKTRACE=0 to act as if unset

**UPDATE:** `RUST_BACKTRACE=0` to act as if the env. var is unset! (now `0` is what `disabled` was for, below)

When RUST_BACKTRACE is set to "disabled" then this acts as if the env. var is unset. So, either make sure `RUST_BACKTRACE` is not set OR set it to `disabled` to achieve the same effect.

Sample usage:

```bash
$ rustc -o /tmp/a.out -- <(echo 'fn main(){ panic!() }') && RUST_BACKTRACE=disabled /tmp/a.out
!! executing '/home/zazdxscf/build/1nonpkgs/rust/rust//x86_64-unknown-linux-gnu/stage2/bin//rustc' with args: '-o /tmp/a.out -- /dev/fd/63'
thread '<main>' panicked at 'explicit panic', /dev/fd/63:1
note: Run with `RUST_BACKTRACE=1` for a backtrace.

$ rustc -o /tmp/a.out -- <(echo 'fn main(){ panic!() }') && RUST_BACKTRACE=1 /tmp/a.out
!! executing '/home/zazdxscf/build/1nonpkgs/rust/rust//x86_64-unknown-linux-gnu/stage2/bin//rustc' with args: '-o /tmp/a.out -- /dev/fd/63'
thread '<main>' panicked at 'explicit panic', /dev/fd/63:1
stack backtrace:
   1:     0x55709e8148c0 - sys::backtrace::tracing::imp::write::h140f24a0cfc189b98Ru
   2:     0x55709e816a5b - panicking::default_hook::_$u7b$$u7b$closure$u7d$$u7d$::closure.45165
   3:     0x55709e8166e8 - panicking::default_hook::hed419823688cb82aXoA
   4:     0x55709e810fff - sys_common::unwind::begin_unwind_inner::hbb9642f6e212d56fmHt
   5:     0x55709e810513 - sys_common::unwind::begin_unwind::h16232867470678019594
   6:     0x55709e810489 - main::hb524f9576270962feaa
   7:     0x55709e816314 - sys_common::unwind::try::try_fn::h1274188004693518534
   8:     0x55709e813dfb - __rust_try
   9:     0x55709e815dab - rt::lang_start::h712b1cd650781872ahA
  10:     0x55709e810679 - main
  11:     0x7efd1026859f - __libc_start_main
  12:     0x55709e810348 - _start
  13:                0x0 - <unknown>
```

Some programs(eg. [vim's syntactic](https://github.com/scrooloose/syntastic) used by [rust.vim](https://github.com/rust-lang/rust.vim)) cannot unset the env. var RUST_BACKTRACE if it's already set(eg. in .bashrc) but [they can set it to some value](https://github.com/respeccing/gentooskyline/blob/cb5533e1598f871d3fdf7c3d8248ce767b5b9360/system/Z575/OSes/gentoo/on_baremetal/filesystem_now/gentoo/home/zazdxscf/build/1nonpkgs/rust.vim/upd#L17), and I needed to ensure the env. var is unset in order to avoid this issue: https://github.com/rust-lang/rust/issues/29293

**EDIT:** Sample usage 2:

```bash
$ export RUST_BACKTRACE=1

$ rustc -o /tmp/a.out -- <(echo 'fn main(){ panic!() }') && /tmp/a.out
!! executing '/home/zazdxscf/build/1nonpkgs/rust/rust//x86_64-unknown-linux-gnu/stage2/bin//rustc' with args: '-o /tmp/a.out -- /dev/fd/63'
thread '<main>' panicked at 'explicit panic', /dev/fd/63:1
stack backtrace:
   1:     0x55c2696738c0 - sys::backtrace::tracing::imp::write::h140f24a0cfc189b98Ru
   2:     0x55c269675a5b - panicking::default_hook::_$u7b$$u7b$closure$u7d$$u7d$::closure.45165
   3:     0x55c2696756e8 - panicking::default_hook::hed419823688cb82aXoA
   4:     0x55c26966ffff - sys_common::unwind::begin_unwind_inner::hbb9642f6e212d56fmHt
   5:     0x55c26966f513 - sys_common::unwind::begin_unwind::h16023941661074805588
   6:     0x55c26966f489 - main::hb524f9576270962feaa
   7:     0x55c269675314 - sys_common::unwind::try::try_fn::h1274188004693518534
   8:     0x55c269672dfb - __rust_try
   9:     0x55c269674dab - rt::lang_start::h712b1cd650781872ahA
  10:     0x55c26966f679 - main
  11:     0x7f593d58459f - __libc_start_main
  12:     0x55c26966f348 - _start
  13:                0x0 - <unknown>

$ rustc -o /tmp/a.out -- <(echo 'fn main(){ panic!() }') && RUST_BACKTRACE=disabled /tmp/a.out
!! executing '/home/zazdxscf/build/1nonpkgs/rust/rust//x86_64-unknown-linux-gnu/stage2/bin//rustc' with args: '-o /tmp/a.out -- /dev/fd/63'
thread '<main>' panicked at 'explicit panic', /dev/fd/63:1
note: Run with `RUST_BACKTRACE=1` for a backtrace.

```

Add functions to convert IPv6 addresses from and to octets

See also #32313.

Rollup of 11 pull requests

- Successful merges: #32622, #32629, #32640, #32641, #32642, #32645, #32647, #32649, #32652, #32654, #32656
- Failed merges:

https://github.com/rust-lang/rust/issues/32551

https://github.com/rust-lang/rust/issues/32551

Remove incorrect comment about `PartialEq` implementation

Fix a typo in the doc comment of `std::os::unix::thread::JoinHandleExt`

Added missing refcell ref/refmut coercions to unsized

Ref/RefMut should be coercible to unsized.
This commit adds a unit test and two missing CoerceUnsized implementations.

rustbuild: Fix dist for non-host targets

The `rust-std` package that we produce is expected to have not only the standard
library but also libtest for compiling unit tests. Unfortunately this does not
currently happen due to the way rustbuild is structured.

There are currently two main stages of compilation in rustbuild, one for the
standard library and one for the compiler. This is primarily done to allow us to
fill in the sysroot right after the standard library has finished compiling to
continue compiling the rest of the crates. Consequently the entire compiler does
not have to explicitly depend on the standard library, and this also should
allow us to pull in crates.io dependencies into the build in the future because
they'll just naturally build against the std we just produced.

These phases, however, do not represent a cross-compiled build. Target-only
builds also require libtest, and libtest is currently part of the
all-encompassing "compiler build". There's unfortunately no way to learn about
just libtest and its dependencies (in a great and robust fashion) so to ensure
that we can copy the right artifacts over this commit introduces a new build
step, libtest.

The new libtest build step has documentation, dist, and link steps as std/rustc
already do. The compiler now depends on libtest instead of libstd, and all
compiler crates can now assume that test and its dependencies are implicitly
part of the sysroot (hence explicit dependencies being removed). This makes the
build a tad less parallel as in theory many rustc crates can be compiled in
parallel with libtest, but this likely isn't where we really need parallelism
either (all the time is still spent in the compiler).

All in all this allows the `dist-std` step to depend on both libstd and libtest,
so `rust-std` packages produced by rustbuild should start having both the
standard library and libtest.

Closes #32523

The `rust-std` package that we produce is expected to have not only the standard
library but also libtest for compiling unit tests. Unfortunately this does not
currently happen due to the way rustbuild is structured.

There are currently two main stages of compilation in rustbuild, one for the
standard library and one for the compiler. This is primarily done to allow us to
fill in the sysroot right after the standard library has finished compiling to
continue compiling the rest of the crates. Consequently the entire compiler does
not have to explicitly depend on the standard library, and this also should
allow us to pull in crates.io dependencies into the build in the future because
they'll just naturally build against the std we just produced.

These phases, however, do not represent a cross-compiled build. Target-only
builds also require libtest, and libtest is currently part of the
all-encompassing "compiler build". There's unfortunately no way to learn about
just libtest and its dependencies (in a great and robust fashion) so to ensure
that we can copy the right artifacts over this commit introduces a new build
step, libtest.

The new libtest build step has documentation, dist, and link steps as std/rustc
already do. The compiler now depends on libtest instead of libstd, and all
compiler crates can now assume that test and its dependencies are implicitly
part of the sysroot (hence explicit dependencies being removed). This makes the
build a tad less parallel as in theory many rustc crates can be compiled in
parallel with libtest, but this likely isn't where we really need parallelism
either (all the time is still spent in the compiler).

All in all this allows the `dist-std` step to depend on both libstd and libtest,
so `rust-std` packages produced by rustbuild should start having both the
standard library and libtest.

Closes #32523

eternally_available linkage isn't legal for declarations anyway, so the
check for an externally_available declaration should never succeed, so
let's remove it.

Register new FreeBSD snapshot

Actual file is at https://people.freebsd.org/~asomers/rust/rust-stage0-2016-03-18-235d774-freebsd-x86_64-390b9a9f60f3d0d6a52c04d939a0355f572d03b3.tar.bz2

Fix formatting in the documentation for AtomicIsize::compare_exchange_weak

Improve E0277 error message in a generic context

This now mentions that you can restrict type parameters to be able to
call functions of traits.