Auto merge of #127388 - compiler-errors:elaboration-tweaks, r=lcnr

Elaboration tweaks

Removes `Filter::OnlySelfThatDefines` and reimplements `transitive_bounds_that_define_assoc_item` as a separate function, since I don't want to have to uplift that mode since it's both an implementation detail (only exists to avoid cycles in astconv) and requires exposing `Ident` as an associated type on `Interner`.

r? lcnr

Author: bors