Reenable the MergeFunctions pass

The crash that happened in #23566 doesn't happen anymore with the LLVM mergefunc
pass enabled and it hugely reduces code size (for example it shaves off 10% of the
final Servo executable). This patch reenables it.

For those wondering, [here are the docs from LLVM about this pass](http://llvm.org/docs/MergeFunctions.html).

Implement label break value (RFC 2046)

Implement label-break-value (#48594).

No error checking or feature gating yet

Review proc macro API 1.2

cc https://github.com/rust-lang/rust/issues/38356

Summary of applied changes:
- Documentation for proc macro API 1.2 is expanded.
- Renamed APIs: `Term` -> `Ident`, `TokenTree::Term` -> `TokenTree::Ident`, `Op` -> `Punct`, `TokenTree::Op` -> `TokenTree::Punct`, `Op::op` -> `Punct::as_char`.
- Removed APIs: `Ident::as_str`, use `Display` impl for `Ident` instead.
- New APIs (not stabilized in 1.2): `Ident::new_raw` for creating a raw identifier (I'm not sure `new_x` it's a very idiomatic name though).
- Runtime changes:
    - `Punct::new` now ensures that the input `char` is a valid punctuation character in Rust.
    - `Ident::new` ensures that the input `str` is a valid identifier in Rust.
    - Lifetimes in proc macros are now represented as two joint tokens - `Punct('\'', Spacing::Joint)` and `Ident("lifetime_name_without_quote")` similarly to multi-character operators.
- Stabilized APIs: None yet.

A bit of motivation for renaming (although it was already stated in the review comments):
- With my compiler frontend glasses on `Ident` is the single most appropriate name for this thing, *especially* if we are doing input validation on construction. `TokenTree::Ident` effectively wraps `token::Ident` or `ast::Ident + is_raw`, its meaning is "identifier" and it's already named `ident` in declarative macros.
- Regarding `Punct`, the motivation is that `Op` is actively misleading. The thing doesn't mean an operator, it's neither a subset of operators (there is non-operator punctuation in the language), nor superset (operators can be multicharacter while this thing is always a single character). So I named it `Punct` (first proposed in [the original RFC](https://github.com/rust-lang/rfcs/pull/1566), then [by @SimonSapin](https://github.com/rust-lang/rust/issues/38356#issuecomment-276676526)) , together with input validation it's now a subset of ASCII punctuation character category (`u8::is_ascii_punctuation`).

Implement RFC 2056 trivial constraints in where clauses

This is an implementation of the new behaviour for #48214. Tests are mostly updated to show the effects of this. Feature gate hasn't been added yet.

Some things that are worth noting and are maybe not want we want

* `&mut T: Copy` doesn't allow as much as someone might expect because there is often an implicit reborrow.
* ~There isn't a check that a where clause is well-formed any more, so `where Vec<str>: Debug` is now allowed (without a `str: Sized` bound).~

r? @nikomatsakis

Update RLS and Rustfmt

Fixes RLS tests

r? @alexcrichton

 Remove ScopeTarget and LoopIdResult

* Remove ScopeTarget in preparation of label-break-value (PR #50045)
* Replace LoopIdResult by Result which is possible now thanks to commit 8ac65af8 " Implement Encodable and Decodable for Result."

r? @EddyB

rustdoc: replace most (e)println! statements with structured warnings/errors

Turns out, the rustc diagnostic handler doesn't need a whole lot of setup that we weren't already doing. For errors that occur outside a "dealing with source code" context, we can just use the format/color config we were already parsing and make up a `Handler` that we can emit structured warnings/errors from. So i did that. This will make it way easier to test things with `rustdoc-ui` tests, since those require the JSON error output. (In fact, this PR is a yak shave for a different one where i was trying to do just that. `>_>`)

The Great Generics Generalisation: Ty Edition

Part of the generic parameter refactoring effort, split off from https://github.com/rust-lang/rust/pull/48149. Contains the `ty`-relative refactoring.

r? @EddyB

Remove unnecessary proc-macro-related `feature`s

Don't inject clippy into the rls anymore

r? @kennytm

sorry about breaking nightlies.

The issue is that the `[patch.crates-io]` doesn't work if the versions differ. So every time we update clippy, we can only update it to the verison that rls is depending on.

I'm disabling the injection of clippy into rls for now. I'm not sure how to do this properly. We could

* add a version check, so rls only builds clippy if its dependency clippy is the same as the submodule clippy
* do something crazy like auto-patching the Cargo.toml of the rls tool repo to just use a path dependency on clippy
* build crates-io clippy instead of submodule clippy and gate clippy injection on that
    * that's somewhat automatic, and is essentially what is necessary right now, but done manually
* make clippy 0.1.* instead of 0.0.* and update patch versions for nightly updates and minor version updates for `clippy_lints` api changes.
    * not sure how semver-great this is

The crash that happened in #23566 doesn't happen anymore with the LLVM mergefunc
pass enabled and it hugely reduces code size (for example it shaves off 10% of the
final Servo executable). This patch reenables it.

https://github.com/rust-lang/llvm/pull/111
https://github.com/rust-lang/llvm/pull/114

It's redundant as Result already implements Encodable
as well as Decodable.

When we want to implement label-break-value,
we can't really decide whether to emit ScopeTarget::Loop or
ScopeTarget::Block in the code that is supposed to create it.
So we get rid of it and reconstruct the information when
needed.