refactor builtin unsize handling, extend comments

r? `@compiler-errors`

Check lazy type aliases for well-formedness

Previously we didn't check if `T: Mul` holds given lazy `type Alias<T> = <T as Mul>::Output;`.
Now we do. It only makes sense.

`@rustbot` label F-lazy_type_alias

r? `@oli-obk`

…ds, r=spastorino

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

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 rust-lang#108304 (1.) and also remove a bunch of match arms (2.).

Fixes rust-lang#108304

r? `@spastorino`

…lor-24, r=notriddle

Migrate GUI colors test to original CSS color format

Follow-up of rust-lang#111459.

r? `@notriddle`

Add missing feature gate in multiple_supertrait_upcastable doc

Fix rust-lang#112424