Allow writing metadata without llvm

# Todo:

* [x] Rebase
* [x] Fix eventual errors
* [x] <strike>Find some crate to write elf files</strike> (will do it later)

Cc #43842

encode region::Scope using fewer bytes

Now that region::Scope is no longer interned, its size is more important. This PR encodes region::Scope in 8 bytes instead of 12, which should speed up region inference somewhat (perf testing needed) and should improve the margins on #36799 by 64MB (that's not a lot, I did this PR mostly to speed up region inference).

This is a perf-sensitive PR. Please don't roll me up.

r? @EddyB

This is based on  #44743 so I could get more accurate measurements on #36799.

Compress most of spans to 32 bits

As described in https://internals.rust-lang.org/t/rfc-compiler-refactoring-spans/1357/28

Closes https://github.com/rust-lang/rust/issues/15594
r? @michaelwoerister

The Magenta kernel is now called Zircon.

 * Adjust bootstrap to provide useful output on failure
 * Add missing package dependencies in the build environment
 * Fix permission bits on prebuilt toolchain files

Move effect-checking to MIR

This allows emitting lints from MIR and moves the effect-checking pass to work on it.

I'll make `repr(packed)` misuse unsafe in a separate PR.

r? @EddyB

This appears to be needed by the Rust build.

This toolchain is already used to build Zircon itself and is the
official Clang toolchain used by all Fuchsia developers.

compiler-rt is now being built as part of the toolchain itself.

put empty generic lists behind a pointer

This reduces the size of hir::Expr from 128 to 88 bytes (!) and shaves
200MB out of #36799.

This is a performance-sensitive PR so please don't roll it up.

r? @EddyB

typeck::check::coercion - roll back failed unsizing type vars

This wraps unsizing coercions within an additional level of
`commit_if_ok`, which rolls back type variables if the unsizing coercion
fails. This prevents a large amount of type-variables from accumulating
while type-checking a large function, e.g. shaving 2GB off one of the
4GB peaks in #36799.

This is a performance-sensitive PR so please don't roll it up.

r? @EddyB
cc @nikomatsakis

Now that region::Scope is no longer interned, its size is more
important. This PR encodes region::Scope in 8 bytes instead of 12, which
should speed up region inference somewhat (perf testing needed) and
should improve the margins on #36799 by 64MB (that's not a lot, I did
this PR mostly to speed up region inference).

Rollup of 4 pull requests

- Successful merges: #44103, #44625, #44789, #44795
- Failed merges:

MIR borrowck: Add span labels for E0381 and E0505

Corresponds to `report_use_of_moved` and `report_move_out_when_borrowed`.

Part of #44596.

Fix warning position in rustdoc code blocks

Before:

<img width="1440" alt="screen shot 2017-09-23 at 14 07 08" src="https://user-images.githubusercontent.com/3050060/30773382-b9649288-a06f-11e7-94ec-faa3c3ed999b.png">

After:

<img width="1440" alt="screen shot 2017-09-23 at 14 58 31" src="https://user-images.githubusercontent.com/3050060/30773384-bdfc9f3e-a06f-11e7-9030-9fb8a5308668.png">

r? @QuietMisdreavus

Indicate how ChildStd{in,out,err} FDs are closed.

Fixes https://github.com/rust-lang/rust/issues/41452.

add comparison operators to must-use lint (under `fn_must_use` feature)

Although RFC 1940 is about annotating functions with `#[must_use]`, a
key part of the motivation was linting unused equality operators.

(See
https://github.com/rust-lang/rfcs/pull/1812#issuecomment-265695898—it
seems to have not been clear to discussants at the time that marking the
comparison methods as `must_use` would not give us the lints on
comparison operators, at least in (what the present author understood
as) the most straightforward implementation, as landed in #43728
(3645b062).)

To rectify the situation, we here lint unused comparison operators as
part of the unused-must-use lint (feature gated by the `fn_must_use`
feature flag, which now arguably becomes a slight (tolerable in the
opinion of the present author) misnomer).

This is in the matter of #43302.

cc @crumblingstatue

this should produce more error stability

No functional changes intended.

This wraps unsizing coercions within an additional level of
`commit_if_ok`, which rolls back type variables if the unsizing coercion
fails. This prevents a large amount of type-variables from accumulating
while type-checking a large function, e.g. shaving 2GB off one of the
4GB peaks in #36799.

This reduces the size of hir::Expr from 128 to 88 bytes (!) and shaves
200MB out of #36799.

Improve diagnostics when attempting to match tuple enum variant with struct pattern

Adds an extra note as below to explain that a tuple pattern was probably intended.

```
error[E0026]: variant `X::Y` does not have a field named `data`
  --> src/main.rs:18:16
   |
18 |         X::Y { data } => println!("The data is {}", data)
   |                ^^^^ variant `X::Y` does not have field `data`

error[E0027]: pattern does not mention field `0`
  --> src/main.rs:18:9
   |
18 |         X::Y { data } => println!("The data is {}", data)
   |         ^^^^^^^^^^^^^ missing field `0`
   |
   = note: trying to match a tuple variant with a struct variant pattern
```

Fixes #41314.