Rollup merge of #125786 - compiler-errors:fold-item-bounds, r=lcnr

Fold item bounds before proving them in `check_type_bounds` in new solver

Vaguely confident that this is sufficient to prevent rust-lang/trait-system-refactor-initiative#46 and rust-lang/trait-system-refactor-initiative#62.

This is not the "correct" solution, but will probably suffice until coinduction, at which point we implement the right solution (`check_type_bounds` must prove `Assoc<...> alias-eq ConcreteType`, normalizing requires proving item bounds).

r? lcnr

Author: matthiaskrgr