Map RPITIT's opaque type bounds back from projections to opaques

[compiler-errors]

An RPITIT in a program's AST is eventually translated into both a projection GAT and an opaque. The opaque is used for default trait methods, like:

trait Foo {
  fn bar() -> impl Sized { 0i32 }
}

The item bounds for both the projection and opaque are identical, and both have a projection self ty. This is mostly okay, since we can normalize this projection within the default trait method body to the opaque, but it does two things:

1. it leads to bugs in places where we don't normalize item bounds, like deduce_future_output_from_obligations
2. it leads to extra match arms that are both suspicious looking and also easy to miss

This PR maps the opaque type bounds of the RPITIT's opaque back to the opaque's self type to avoid this quirk. Then we can fix the UI test for #108304 (1.) and also remove a bunch of match arms (2.).

Fixes #108304

r? @spastorino

[spastorino]

@bors r+

[bors]

📌 Commit 6f5c51f has been approved by spastorino

It is now in the queue for this repository.