Support `Self` in struct expressions and patterns
Struct expressions and patterns generally support type aliases `Alias { field: 10 }` i.e. they already have to work with `ty::Ty` to do their job. `Self` is a type alias (when it's not a type parameter) => struct expressions and patterns should support `Self`.
Typical example:
```
impl MyStruct {
fn new() -> Self {
Self { a: 10, b: "Hello" }
}
}
```
The first commit does some preparations and cleanups, see the commit message for details.
This also fixes couple of bugs related to aliases in struct paths (fixes https://github.com/rust-lang/rust/issues/36286).
EDIT:
Since struct expressions and patterns always work with `ty::Ty` now, associated paths in them are also supported. If associated type `A::B` successfully resolves to a struct (or union) type, then `A::B { /* fields */ }` is a valid expression/pattern. This will become more important when enum variants are treated as [associated items](https://github.com/rust-lang/rust/issues/26264#issuecomment-250603946).
r? @eddyb
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