Rollup merge of #139564 - compiler-errors:deeply-norm, r=lcnr

Deeply normalize obligations in `BestObligation` folder

Built on #139513.

This establishes a somewhat rough invariant that the `Obligation`'s predicate is always deeply normalized in the folder; when we construct a new obligation we normalize it.

Putting this up for discussion since it does affect some goals.

r? lcnr

Author: Zalathar