libstdc++: Make std::unique_lock self-move-assignable

libstdc++: Make std::unique_lock self-move-assignable

LWG 4172 was approved in Hagenberg, February 2025, fixing
std::unique_lock and std::shared_lock to work correctly for
self-move-assignment.

Our std::shared_lock was already doing the right thing (contradicting
the standard) so just add a comment there. Our std::unique_lock needs to
be fixed to do the right thing.

libstdc++-v3/ChangeLog:

	* include/bits/unique_lock.h (unique_lock::operator=): Fix for
	self-move-assignment.
	* include/std/shared_mutex (shared_lock::operator=): Add
	comment.
	* testsuite/30_threads/shared_lock/cons/lwg4172.cc: New test.
	* testsuite/30_threads/unique_lock/cons/lwg4172.cc: New test.

Reviewed-by: Patrick Palka <ppalka@redhat.com>