GAT where bounds of impl candidates

[lcnr]

we register them since rust-lang/rust#113336, but they may already be proven by wf-bounds so it is potentially unnecessary?

wf-bounds skip escaping bound vars, so we probably should.

affects both inherent assoc types and ordinary assoc types. The new solver now checks them in probe_and_match_goal_against_assumption and for impl candidates. TODO: which candidates don't check GAT where-clauses rn