This fixes the bug we previously had where we'd build a libtest tool
after building a libstd tool and clear out the libstd tool. Since we
clear out all tools for a given stage on invocations of CleanTools after
lib{std, test, rustc} change, we need to make sure that all tools built
with that stage will be built after the clearing is done.

The fix contained here technically isn't perfect; there is still an edge
case of compiling a libstd tool, then compiling libtest, which will
clear out the libstd tool and it won't ever get rebuilt within that
session of rustbuild. This is where the caching system used today shows
it's problems -- in effect, all tools depend on a global counter of the
stage being cleared out. We can implement such a counter in a future
patch to ensure that tools are rebuilt as needed, but it is deemed
unlikely that it will be required in practice, since most if not all
tools are built after the relevant stage's std/test/rustc are built,
though this is only an opinion and hasn't been verified.

This permits proc macro crates to correctly work with rustdoc.

Some users of the build system change the git sha on every build due to
utilizing git to push changes to a remote server. This allows them to
simply configure that away instead of depending on custom patches to
rustbuild.

We first check the configuration, then passed parameters (--build), then
fall back to the auto-detection that bootstrap.py does.

Fixes #39673.

We no longer care about the source of this information, so there is no
reason to restrict users.

This introduces a slight change in behavior, where we unilaterally
respect the --host and --target parameters passed for all sanity
checking and runtime configuration.

Improve std::ops docs

Fixes #29365. (This fixes all but one point from @steveklabnik's list, but that point was referring to examples of implementing range traits, but there are no range traits in std::ops.)

The main changes are quite a bit of copyediting, adding more "real" examples for some of the traits, incorporating some guidance from the API docs, more linking (cross-docs and to the book & reference), cleaning up examples, moving things around, and so on. Refer to the commit messages for more details.

Note: I decided to link to the second edition of the book since I think it's more appropriate now for the sections I linked, if this is not okay, please say so!

* fixed link typos and copy-paster errors
* rewrote Fn* explanations
* `RHS = Self` -> `RHS` is `Self` (added that to all applicable places as
  well)
* fixed up some links
* s/MutDeref/DerefMut
* removed remaining superfluous `fn main()`s
* fixed some minor phrasings and factual errors and inaccuracies

std::ops docs: Fix phrasing and factual errors/inaccuracies

Move config.toml.example to the root dir

It's way more discoverable here.

r? @alexcrichton

Update GitHub pull request documentation link

It looks like the GitHub documentation has been re-organized so that the "fork and pull" model isn't explained alongside the PR process in one convenient link any more.

Check #[thread_local] statics correctly in the compiler.

Fixes #43733 by introducing `#[allow_internal_unsafe]` analogous to `#[allow_internal_unstable]`, for letting a macro expand to `unsafe` blocks and functions even in `#![forbid(unsafe_code)]` crates.

Fixes #17954 by not letting references to `#[thread_local]` statics escape the function they're taken in - we can't just use a magical lifetime because Rust has *lifetime parametrism*, so if we added the often-proposed `'thread` lifetime, we'd have no way to check it in generic code.
To avoid potential edge cases in the compiler, the lifetime is actually that of a temporary at the same position, i.e. `&TLS_STATIC` has the same lifetime `&non_const_fn()` would.

Referring to `#[thread_local]` `static`s at compile-time is banned now (as per PR discussion).

Additionally, to remove `unsafe impl Sync` from `std::thread::local::fast::Key`, `#[thread_local]` statics are now not required to implement `Sync`, as they are not shared between threads.

rustdoc doc tests

This is mostly adapted from the book v1's chapter.

cc https://github.com/rust-lang/rust/issues/42322

r? @rust-lang/docs

Fix some typos

I wrote a really naive script and found those typos in the documentation.

It's way more discoverable here.

Document the doc attribute

cc #42322

For box expressions, use NZ drop instead of a free block

This falls naturally out of making drop elaboration work with `box`
expressions, which is probably required for sane MIR borrow-checking.
This is a pure refactoring with no intentional functional effects.

r? @nagisa

Improve LLVM/trans scheduling a bit

Currently it's possible that the main thread is waiting on LLVM threads to finish work while its implicit token is going to waste. This PR let's the main thread take over, so one of the running LLVM threads can free its token earlier.

r? @alexcrichton

Put `intrinsics::unreachable` on a possible path to stabilization

Mark it with the `unreachable` feature and put it into the `mem` module.
This is a pretty straight-forward API that can already be simulated in
stable Rust by using `transmute` to create an uninhabited enum that can
be matched.

Support compiling rustc without LLVM (try 2)

Now doesn't change rustc_driver.

Supersedes #42752

AddValidation: handle Call terminators into blocks that have multiple incoming edges

The old code was just wrong: It would add validation on paths that don't even come from the call, and it would add multiple validations if multiple calls end return to the same block.

Rollup of 18 pull requests

- Successful merges: #43176, #43632, #43650, #43712, #43715, #43721, #43739, #43741, #43744, #43747, #43752, #43760, #43773, #43779, #43783, #43791, #43793, #43795
- Failed merges:

Improve enum variants display

r? @rust-lang/docs

Before:

<img width="1440" alt="screen shot 2017-08-11 at 00 22 54" src="https://user-images.githubusercontent.com/3050060/29194776-728ce0e2-7e2b-11e7-8299-8300cc0c168b.png">

After:

<img width="1440" alt="screen shot 2017-08-11 at 00 22 57" src="https://user-images.githubusercontent.com/3050060/29194783-78867558-7e2b-11e7-9226-1327fd20163a.png">

(The doc of the variant is more aligned with the "[-]" now).

Fix broken links in Arc documentation

Some link references had ticks, when they should not have had them.

File docs

r? @rust-lang/docs

Fix typo corersponding -> corresponding

I was reading the documentation of __Struct std::os::unix::net::UnixListener__ when I saw a typo inside.

Fix typo in unicode char definition

Reference: http://www.fileformat.info/info/unicode/char/16ed/index.htm

fix a typo

(this should not have been merged with this typo)