libstdc++: Implement LWG 3683 for pmr::polymorphic_allocator

This issue has recently been moved to Tentatively Ready, and seems
uncontroversial. This allows equality comparison with types that are
convertible to pmr::polymorphic_allocator, which fail deduction for the
existing equality operator.

libstdc++-v3/ChangeLog:

	* include/std/memory_resource (polymorphic_allocator): Add
	non-template equality operator, as proposed for LWG 3683.
	* testsuite/20_util/polymorphic_allocator/lwg3683.cc: New test.