Lower constant patterns with ascribed types.
Fixes#57960.
This PR fixes a bug introduced by #55937 which started checking user
type annotations for associated type patterns. Where lowering a
associated constant expression would previously return a
`PatternKind::Constant`, it now returns a `PatternKind::AscribeUserType`
with a `PatternKind::Constant` inside, this PR unwraps that to
access the constant pattern inside and behaves as before.
r? @pnkfelix
a2ec156a5b