Rollup merge of #121702 - compiler-errors:coerce-alias-relate, r=lcnr

Process alias-relate obligations in CoerceUnsized loop

After #119106, we now emit `AliasRelate` goals when relating `?0` and `Alias<T, ..>` in the new solver. In the ad-hoc `CoerceUnsized` selection loop, we now may have `AliasRelate` goals which must be processed to constrain type variables which are mentioned in other goals.

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For example, in the included test, we try to coerce `&<ManuallyDrop<T> as Deref>::Target` to `&dyn Foo`. This requires proving:
* 1 `&<ManuallyDrop<T> as Deref>::Target: CoerceUnsized<&dyn Foo>`
    * 2 `<ManuallyDrop<T> as Deref>::Target alias-relate ?0`
    * 3 `?0: Unsize<dyn Foo>`
        * 4 `?0: Foo`
        * 5 `?0: Sized`

If we don't process goal (2.) before processing goal (3.), then we hit ambiguity since `?0` is never constrained, and therefore we bail out, refusing to coerce the types. After processing (2.), we know `?0 := T`, and the rest of the goals can be processed normally.

Author: GuillaumeGomez