libstdc++: Add warnings for some C++23 deprecations
LWG 3036 deprecates std::pmr::polymorphic_allocator<T>::destroy in
favour of the equivalent member of std::allocator_traits.
LWG 3170 deprecates std::allocator<T>::is_always_equal in favour of
the equivalent member of std::allocator_traits.
This also updates a comment to note that we support the LWG 3541 change
(even before the issue was opened).
Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:
* include/bits/allocator.h (allocator::is_always_equal): Deprecate.
* include/bits/iterator_concepts.h (indirectly_readable_traits):
Add LWG issue number to comment.
* include/std/memory_resource (polymorphic_allocator::release):
Deprecate.
* testsuite/20_util/allocator/requirements/typedefs.cc: Add
dg-warning for deprecation. Also check std::allocator<void>.