Add a MIR-borrowck-only output mode

Removes the `-Z borrowck-mir` flag in favour of a `-Z borrowck=mode` flag where mode can be `mir`, `ast`, or `compare`.

* The `ast` mode represents the current default, passing `-Z borrowck=ast` is equivalent to not passing it at all.
* The `compare` mode outputs both the output of the MIR borrow checker and the AST borrow checker, each error with `(Ast)` and `(Mir)` appended. This mode has the same behaviour as `-Z borrowck-mir` had before this commit.
* The `mir` mode only outputs the results of the MIR borrow checker, while suppressing the errors of the ast borrow checker

The PR also updates the tests to use the new flags.

closes  #46097

rustc_trans: don't apply noalias on returned references.

In #45225 frozen returned `&T` were accidentally maked `noalias`, unlike `&mut T`.
Return value `noalias` is only sound for functions that return dynamic allocations, e.g. `Box`, and using it on anything else can lead to miscompilation, as LLVM assumes certain usage patterns.
Fixes #46239.

where mode is one of {ast,mir,compare}.

This commit only implements the functionality.
The tests will be updated in a follow up commit.

introduce macros for type-foldable and lift, convert stuff to use them

A random commit from a branch I've shelved for the time being that made `TypeFoldable` stuff a bit less annoying to write.

r? @EddyB

Kill the storage for all locals on returning terminators

Fixes #45704.

Rollup of 7 pull requests

- Successful merges: #46201, #46224, #46234, #46252, #46259, #46264, #46269
- Failed merges:

Check tail node in check_links

InstCombine Len([_; N]) => const N in MIR

A small opportunity I noticed in passing.

Not super exciting on its own, but opens the door for a const propagation pass that could completely remove const bounds checks from arrays at MIR time, for example.

Derive Debug for LangItem

Update MSVC compilation instructions regarding path length on Windows

Fix #46214

core::marker fix typo

Remove invalid doc link

r? @rust-lang/docs

Adding `eprint*!` to the list of macros in the `format!` family

The `eprint!` and `eprintln!` macros were added in 76127275. The `std::fmt` documentation does not mention these macros next to `print!` and `println!` in the [Related macros](https://doc.rust-lang.org/std/fmt/#related-macros) section, and I did not find evidence that this omission was deliberate. This PR adds such documentation.

The first modification is to add `eprint!` and `eprintln!` to the list of related macros in the `format!` family. This is how it appears with this change:

![image](https://user-images.githubusercontent.com/933552/33159527-67056caa-cfc8-11e7-8b7d-4224ef2fce4e.png)

The second modification adds a sub-section for `eprint!` and `eprintln!`. Here is how the new section appears:

![image](https://user-images.githubusercontent.com/933552/33159541-97d03bee-cfc8-11e7-8b95-4d3632b5ab7b.png)

Implement `Rc`/`Arc` conversions for string-like types

Provides the following conversion implementations:

* `From<`{`CString`,`&CStr`}`>` for {`Arc`,`Rc`}`<CStr>`
* `From<`{`OsString`,`&OsStr`}`>` for {`Arc`,`Rc`}`<OsStr>`
* `From<`{`PathBuf`,`&Path`}`>` for {`Arc`,`Rc`}`<Path>`

Closes #45008

Provides the following conversion implementations:

* `From<`{`CString`,`&CStr`}`>` for {`Arc`,`Rc`}`<CStr>`
* `From<`{`OsString`,`&OsStr`}`>` for {`Arc`,`Rc`}`<OsStr>`
* `From<`{`PathBuf`,`&Path`}`>` for {`Arc`,`Rc`}`<Path>`

Do match-check for consts

Fixes #43195 (ICE caused by building MIR that contains non-exausitive match)

Be more obvious when suggesting dereference

Include `&` span when suggesting dereference on a span that is already a reference:

```
error: non-reference pattern used to match a reference (see issue #42640)
  --> dont-suggest-dereference-on-arg.rs:16:19
   |
16 |         .filter(|&(ref a, _)| foo(a))
   |                  ^^^^^^^^^^^ help: consider using: `&&(ref k, _)`
   |
   = help: add #![feature(match_default_bindings)] to the crate attributes to enable
```

Fix #45925.

rustc: Add support for some more x86 SIMD ops

This commit adds compiler support for two basic operations needed for binding
SIMD on x86 platforms:

* First, a `nontemporal_store` intrinsic was added for the `_mm_stream_ps`, seen
  in rust-lang-nursery/stdsimd#114. This was relatively straightforward and is
  quite similar to the volatile store intrinsic.

* Next, and much more intrusively, a new type to the backend was added. The
  `x86_mmx` type is used in LLVM for a 64-bit vector register and is used in
  various intrinsics like `_mm_abs_pi8` as seen in rust-lang-nursery/stdsimd#74.
  This new type was added as a new layout option as well as having support added
  to the trans backend. The type is enabled with the `#[repr(x86_mmx)]`
  attribute which is intended to just be an implementation detail of SIMD in
  Rust.

I'm not 100% certain about how the `x86_mmx` type was added, so any extra eyes
or thoughts on that would be greatly appreciated!

This commit adds compiler support for two basic operations needed for binding
SIMD on x86 platforms:

* First, a `nontemporal_store` intrinsic was added for the `_mm_stream_ps`, seen
  in rust-lang-nursery/stdsimd#114. This was relatively straightforward and is
  quite similar to the volatile store intrinsic.

* Next, and much more intrusively, a new type to the backend was added. The
  `x86_mmx` type is used in LLVM for a 64-bit vector register and is used in
  various intrinsics like `_mm_abs_pi8` as seen in rust-lang-nursery/stdsimd#74.
  This new type was added as a new layout option as well as having support added
  to the trans backend. The type is enabled with the `#[repr(x86_mmx)]`
  attribute which is intended to just be an implementation detail of SIMD in
  Rust.

I'm not 100% certain about how the `x86_mmx` type was added, so any extra eyes
or thoughts on that would be greatly appreciated!

rustbuild: Enable WebAssembly backend by default

This commit alters how we compile LLVM by default enabling the WebAssembly
backend. This then also adds the wasm32-unknown-unknown target to get compiled
on the `cross` builder and distributed through rustup. Tests are not yet enabled
for this target but that should hopefully be coming soon!

This commit alters how we compile LLVM by default enabling the WebAssembly
backend. This then also adds the wasm32-unknown-unknown target to get compiled
on the `cross` builder and distributed through rustup. Tests are not yet enabled
for this target but that should hopefully be coming soon!

rustc: don't mark lifetimes as early-bound in the presence of impl Trait.

This hack from the original implementation shouldn't be needed anymore, thanks to @cramertj.

r? @nikomatsakis

Properly handle reexport of foreign items.

Handles `pub use` of `extern { fn, static, type }`. Also plug in some more `match` arms where handling `extern type` is reasonable.

Fixed #46098.

allocators: don’t assume MIN_ALIGN for small sizes

See individual commit messages.

Fix path search

Fixes #46015.

r? @QuietMisdreavus

rustbuild: Update LLVM and enable ThinLTO

This commit updates LLVM to fix #45511 (https://reviews.llvm.org/D39981) and
also reenables ThinLTO for libtest now that we shouldn't hit #45768. This also
opportunistically enables ThinLTO for libstd which was previously blocked
(#45661) on test failures related to debuginfo with a presumed cause of #45511.

Closes #45511

Turns out ThinLTO was internalizing this symbol and eliminating it. Worse yet if
you compiled with LTO turns out no TLS destructors would run on Windows! The
`#[used]` annotation should be a more bulletproof implementation (in the face of
LTO) of preserving this symbol all the way through in LLVM and ensuring it makes
it all the way to the linker which will take care of it.

incr.comp.: Make sure we don't lose unused green results from the query cache.

In its current implementation, the query result cache works by bulk-writing the results of all cacheable queries into a monolithic binary file on disk. Prior to this PR, we would potentially lose query results during this process because only results that had already been loaded into memory were serialized. In contrast, results that were not needed during the given compilation session were not serialized again.

This PR will do one pass over all green `DepNodes` that represent a cacheable query and execute the corresponding query in order to make sure that the query result gets loaded into memory before cache serialization.

In the future we might want to look into a serialization format the can be updated in-place so that we don't have to load unchanged results just for immediately storing them again.

r? @nikomatsakis