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.
(cherry picked from commit
f13f9c99dbee9c495955a430dd10bdb24a16f24a)