Improve diagnostics for non-well-formed associated items

[ozankenangungor]

Fixes #162368.

With the new solver, this ended up as a projection mismatch instead of reporting the Sized requirement from the associated type.

I changed the diagnostic fallback to also check the associated item's own clauses, so it can recover the Self: Sized obligation and report E0277 instead.

In this case, the diagnostic now points to the impl type parameter instead of i32, keeps the note for the associated type bound, and suggests adding where Self: Sized.

I also added regression tests for the original case, an existing where-clause, the normal implicit Sized case, and a non-impl projection use site.

[rustbot]

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

[rustbot]

r? @ShoyuVanilla

rustbot has assigned @ShoyuVanilla.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 75 candidates
• Random selection from 20 candidates