Auto merge of #45047 - durka:trait-alias, r=petrochenkov

trait alias infrastructure

This will be an implementation of trait aliases (RFC 1733, #41517).

Progress so far:

- [x] Feature gate
- [x] Add to parser
  - [x] `where` clauses
    - [x] prohibit LHS type parameter bounds via AST validation https://github.com/rust-lang/rust/pull/45047#discussion_r143575575
- [x] Add to AST and HIR
  - [x] make a separate PathSource for trait alias contexts https://github.com/rust-lang/rust/pull/45047#discussion_r143353932
- [x] Stub out enough of typeck and resolve to just barely not ICE

Postponed:

- [ ] Actually implement the alias part
- [ ] #21903
- [ ] #24010

I need some pointers on where to start with that last one. The test currently does this:

```
error[E0283]: type annotations required: cannot resolve `_: CD`
  --> src/test/run-pass/trait-alias.rs:34:16
   |
34 |     let both = foo();
   |                ^^^
   |
   = note: required by `foo`
```

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