libstdc++: Specialize allocator_traits<pmr::polymorphic_allocator<T>>

libstdc++: Specialize allocator_traits<pmr::polymorphic_allocator<T>>

This adds a partial specialization of allocator_traits, similar to what
was already done for std::allocator. This means that most uses of
polymorphic_allocator via the traits can avoid the metaprogramming
overhead needed to deduce the properties from polymorphic_allocator.

In addition, I'm changing polymorphic_allocator::delete_object to invoke
the destructor (or pseudo-destructor) directly, rather than calling
allocator_traits::destroy, which calls polymorphic_allocator::destroy
(which is deprecated). This is observable if a user has specialized
allocator_traits<polymorphic_allocator<Foo>> and expects to see its
destroy member function called. I consider explicit specializations of
allocator_traits to be wrong-headed, and this use case seems unnecessary
to support. So delete_object just invokes the destructor directly.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/std/memory_resource (polymorphic_allocator::delete_object):
	Call destructor directly instead of using destroy.
	(allocator_traits<polymorphic_allocator<T>>): Define partial
	specialization.