Part of rust-lang#29378.

Part of rust-lang#29378 .

- In particular explains *why* we would use the builder instead of
  the free function.
- Changes the parent-child explanation for a spawned-caller.
- Add link to `io::Result` in `thread::Builder`
- Corrects the `thread::Builder::spawn` documentation.

Move the code for loading metadata from rlibs and dylibs from
rustc_metadata into rustc_trans, and introduce a trait to avoid
introducing a direct dependency on rustc_trans.

This means rustc_metadata is no longer rebuilt when LLVM changes.

Should help deal with spurious download failures.

Make diagnostic note for existing method with unsatisfied trait bounds
multiline for cleaner output.

```
  = note: the method `count` exists but the following trait bounds were not satisfied:
          `[closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53] : std::ops::FnMut<(&_,)>`
          `std::iter::Filter<std::iter::Fuse<std::iter::Once<&str>> [closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53]> : std::iter::Iterator`

Before:

```
  = note: the method `count` exists but the following trait bounds were not satisfied: `[closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53] : std::ops::FnMut<(&_,)>`, `std::iter::Filter<std::iter::Fuse<std::iter::Once<&str>>, [closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53]> : std::iter::Iterator`
```

Part of rust-lang#29378 .

Consequently, session creation can no longer initialize LLVM.
The few places that use the compiler without going through
rustc_driver/CompilerCalls thus need to be careful to manually
initialize LLVM (via rustc_trans!) immediately after session
creation.

This means librustc is not rebuilt when LLVM changes.

This does not actually improve build times, since it still depends
on rustc_trans, but is better layering and fits the multi-backend
future slightly better.

…r=arielb1

Make unsatisfied trait bounds note multiline

Make diagnostic note for existing method with unsatisfied trait bounds
multiline for cleaner output.

```
  = note: the method `count` exists but the following trait bounds were not satisfied:
          `[closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53] : std::ops::FnMut<(&_,)>`
          `std::iter::Filter<std::iter::Fuse<std::iter::Once<&str>> [closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53]> : std::iter::Iterator`
```

Before:

```
  = note: the method `count` exists but the following trait bounds were not satisfied: `[closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53] : std::ops::FnMut<(&_,)>`, `std::iter::Filter<std::iter::Fuse<std::iter::Once<&str>>, [closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53]> : std::iter::Iterator`
```

Make only rustc_trans depend on rustc_llvm

With these changes, only rustc_trans depends directly on rustc_llvm (and no crate gained a new dependency on trans). This means changing LLVM doesn't rebuild librustc or rustc_metadata, only rustc_trans, rustc_driver and the rustc executable
Also, rustc_driver technically doesn't know about LLVM any more (of course, it still handles a ton of options that conceptually refer to LLVM, but it delegates their implementation to trans).

What I *didn't* implement was merging most or all of rustc_llvm into rustc_trans. I ran into a nasty bug, which was probably just a silly typo somewhere but I probably won't have the time to figure it out in the next week or two. I opened rust-lang#41699 for that step.

Fixes rust-lang#41473

…umeGomez

[Doc] Explain why `thread::yield_now` could be used.

Part of rust-lang#29378.

r? @steveklabnik

…eGomez

[Doc] Implrove `thread::Builder`'s doc.

Part of rust-lang#29378 .

- Explains *why* we would use the builder instead ofthe free function.
- Changes the parent-child explanation for a spawned-caller in `thread::Builder::spawn`
- Adds a link to `io::Result` in `thread::Builder`
- Corrects the return type doc in `thread::Builder::spawn`

r? @rust-lang/docs

[Doc] Add links to the `thread::LocalKey` doc.

Part of rust-lang#29378 .

I do not know exactly what should be done for the `cleanup` part, if you have any idea I'll gladly do it.

r? @rust-lang/docs

…ulacrum

appveyor: Wrap handle.exe download in a retry

Should help deal with spurious download failures.