libstdc++: Fix uses_allocator_construction_args for pair<T&&, U&&> [PR108952]

libstdc++: Fix uses_allocator_construction_args for pair<T&&, U&&> [PR108952]

This implements LWG 3527 which fixes the handling of pair<T&&, U&&> in
std::uses_allocator_construction_args.

libstdc++-v3/ChangeLog:

	PR libstdc++/108952
	* include/bits/uses_allocator_args.h
	(uses_allocator_construction_args): Implement LWG 3527.
	* testsuite/20_util/pair/astuple/get-2.cc: New test.
	* testsuite/20_util/scoped_allocator/108952.cc: New test.
	* testsuite/20_util/uses_allocator/lwg3527.cc: New test.