Auto merge of #135618 - lcnr:coherence-unknown, r=compiler-errors

add cache to `AmbiguityCausesVisitor`

fixes #135457, alternative to #135524.

cc https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/new-solver.20hang.20.23135457

r? `@compiler-errors`