Account for type alias projections in E0308 "expected/found" shortening logic

[estebank]

When encountering a E0308 error involving ty::Projections, highlight only the sub-parts that are actually relevant:

error[E0308]: mismatched types
  --> tests/ui/associated-types/long-type-E0308.rs:19:5
   |
16 | fn foo<'a, T: FooBar>(value: T) -> <<<T::Wrapper<T> as FooBar>::Assoc as FooBar>::Assoc as FooBar>::Wrapper<<T::Assoc as FooBar>::Assoc> {
   |                                    ----------------------------------------------------------------------------------------------------- expected `<<<... as FooBar>::Assoc as FooBar>::Assoc as FooBar>::Wrapper<...>` because of return type
...
19 |     value.assoc().assoc().assoc().wrap(value.assoc().assoc())
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `really_really_really_long_module::FooBar::Wrapper`, found `really_really_really_long_module::FooBar::Assoc`
   |
   = note: expected associated type `<<<<_ as FooBar>::Wrapper<T> as FooBar>::Assoc as FooBar>::Assoc as FooBar>::Wrapper<_>`
              found associated type `<<<<_ as FooBar>::Assoc as FooBar>::Assoc as FooBar>::Assoc as FooBar>::Wrapper<_>`
   = note: an associated type was expected, but a different one was found
   = note: the full name for the type has been written to 'long-type-E0308.long-type-5396540309891820670.txt'
   = note: consider using `--verbose` to print the full type name to the console

Partially address #159043. To close that ticket we also need to modify the type shortening logic to allow for a mechanism to keep relevant sub-types as visible when possible, we need to expand this logic to account for other ty::Aliases, and deduplicate some of the existing highlighting logic.

r? @chenyukang

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.