Remove checks for LLVM < 4.0

While we still have to support LLVM 4.0 for Emscripten, we can drop checks for LLVM >= 4.0 and < 4.0.

enforce unused-must-use lint in macros

Fixes #55516 by turning on the UNUSED_MUST_USE lint within macros.

rustc_target: pass contexts by reference, not value.

`LayoutOf` now takes `&self` instead of `self`, and so does every method generic over a context that implements `LayoutOf` and/or other traits, like `HasDataLayout`, `HasTyCtxt`, etc.

Originally using by-value `Copy` types was relevant because `TyCtxt` was one of those types, but now `TyCtxt::layout_of` is separate from `LayoutOf`, and `TyCtxt` is not an often used layout context.

Passing these context by reference is a lot nicer for miri, which has `self: &mut EvalContext`, and needed `f(&self)` (that is, creating `&&mut EvalContext` references) for layout purposes.
Now, the `&mut EvalContext` can be passed to a function expecting `&C`, directly.

This should help with #54012 / #55627 (to not need `where &'a T::Cx: LayoutOf` bounds).

r? @nikomatsakis or @oli-obk or @nagisa cc @sunfishcode

Rename `Value` to `Immediate` for miri

r? @RalfJung

Move collect_and_partition_mono_items to rustc_mir

Most of the logic of it is inside rustc_mir anyway.

Also removes the single function crate rustc_metadata_utils. Based on #55225

single life

 * structured ~~autofixable~~ (well, pending #53934 and rust-lang-nursery/rustfix#141) suggestions for the single-use-lifetimes lint in the case of function and method reference args
 * don't consider the anonymous lifetime `'_` as "single-use" (it's intended for exactly this sort of thing)

![single_life](https://user-images.githubusercontent.com/1076988/47613227-3b2b6400-da48-11e8-8efd-cb975ddf537d.png)

r? @nikomatsakis

Use token description in "expected/found" parse messages

Fix #54309.

rustdoc: Replaces fn main search and extern crate search with proper parsing during doctests.

Fixes #21299.
Fixes #33731.

Let me know if there's any additional changes you'd like made!

submodules: update clippy from a20599ab to 71ec4ff6

Should fix clippy toolstat.

Changes:

````
rustup https://github.com/rust-lang/rust/pull/55330/
Update stderr
Rename test files
Also lint cfg_attr(.., rustfmt::skip)
Add tests from rustfmt::skip test file
Run update_lints.py script
Add test for non-crate-level inner attributes
Differ between inner and outer attributes
Add tests
Add cfg_attr(rustfmt) lint
Addressed comments.
Fix dogfood error.
Added lints `into_iter_on_ref` and `into_iter_on_array`. Fix #1565.
Allow single_match_else
Update stderr
Add copyright statement©
Fix typos
Fix dogfood error
Fix typo and indentation
run update_lints script
Add tests for unknwon_clippy_lints lint
Add new lint: unknwon_clippy_lintsg
clippy: fix pedantic warnings and run clippy::pedantic lints on the codebase.
Fix a false-positive of needless_borrow
UI test cleanup: Extract match_overlapping_arm tests
UI test cleanup: Extract expect_fun_call tests
Add missing code of conduct file
Use slice patterns instead of padding
Fix dogfood and pedantic lints
ci: when installing rust-toolchain-installer-master, install it in debug mode to save some time in ci.
RIIR update lints: Generate deprecated lints
Replace big if/else expression with match
````

Fixed the bug in bootstrap where --exclude was ignored for run-pass test

This should fix the 3 hour timeout on AppVeyor which happened a lot recently.

Additionally, further rebalanced the AppVeyor subsets by moving "ui" and "linkchecker" into Set 2.

Should fix clippy toolstat.

Changes:

````
rustup https://github.com/rust-lang/rust/pull/55330/
Update stderr
Rename test files
Also lint cfg_attr(.., rustfmt::skip)
Add tests from rustfmt::skip test file
Run update_lints.py script
Add test for non-crate-level inner attributes
Differ between inner and outer attributes
Add tests
Add cfg_attr(rustfmt) lint
Addressed comments.
Fix dogfood error.
Added lints `into_iter_on_ref` and `into_iter_on_array`. Fix #1565.
Allow single_match_else
Update stderr
Add copyright statement©
Fix typos
Fix dogfood error
Fix typo and indentation
run update_lints script
Add tests for unknwon_clippy_lints lint
Add new lint: unknwon_clippy_lintsg
clippy: fix pedantic warnings and run clippy::pedantic lints on the codebase.
Fix a false-positive of needless_borrow
UI test cleanup: Extract match_overlapping_arm tests
UI test cleanup: Extract expect_fun_call tests
Add missing code of conduct file
Use slice patterns instead of padding
Fix dogfood and pedantic lints
ci: when installing rust-toolchain-installer-master, install it in debug mode to save some time in ci.
RIIR update lints: Generate deprecated lints
Replace big if/else expression with match
````

This should allow the timings be more balanced.

Fix the recent spurious 3 hour timeouts.

Implement trait aliases (RFC 1733)

Extends groundwork done in https://github.com/rust-lang/rust/pull/45047, and fully implements https://github.com/rust-lang/rfcs/pull/1733.

CC @durka @nikomatsakis

Rollup of 5 pull requests

Successful merges:

 - #54162 (Hide default impls items)
 - #55555 (Make `-Z ls` list the actual filename of external dependencies)
 - #55567 (add test for deriving Debug on uninhabited enum)
 - #55568 (test that rustdoc doesn't overflow on a big enum)
 - #55598 (publish-toolstate: ping maintainers when a tool builds again)

Failed merges:

r? @ghost

Co-Authored-By: Nbjorn3 <bjorn3@users.noreply.github.com>

publish-toolstate: ping maintainers when a tool builds again

And add @Xanewok as an RLS maintainer

r? @kennytm

Motivation is that I see when the RLS gets broken, but have to poll the website to see when it is fixed, I'd prefer to get pinged.

test that rustdoc doesn't overflow on a big enum

Adds a test to close #25295. The test case depended on `enum_primitive` so I just basically pulled its source into an auxiliary file, is that the right way to do it?

add test for deriving Debug on uninhabited enum

Adds a test to close #38885.

Make `-Z ls` list the actual filename of external dependencies

The hash is pointless for external consumers - extra_filename is the thing that actually gets used, per https://github.com/rust-lang/rust/blob/ca2639e/src/librustc_metadata/locator.rs#L312-L313

Hide default impls items

Follow up of #51885.
Fixes #54025.

cc @Mark-Simulacrum

r? @QuietMisdreavus

And screenshots of course:

<img width="1440" alt="screen shot 2018-09-12 at 23 30 35" src="https://user-images.githubusercontent.com/3050060/45454424-1ff8d500-b6e4-11e8-9257-030322495d58.png">

<img width="1440" alt="screen shot 2018-09-12 at 23 30 42" src="https://user-images.githubusercontent.com/3050060/45454431-2424f280-b6e4-11e8-8d65-db0d85ac18f0.png">

Add support for bound types

This PR may have some slight performance impacts, I don't know how hot is the code I touched.

Also, this breaks clippy and miri.

r? @nikomatsakis

Use the new `BoundVarReplacer` to perform canonical substitutions.