PR libstdc++/70966 make pmr::new_delete_resource() immortal

PR libstdc++/70966 make pmr::new_delete_resource() immortal

Construct the program-wide resource objects using placement new. This
means they have dynamic storage duration and won't be destroyed during
termination.

	PR libstdc++/70966
	* include/experimental/memory_resource (__resource_adaptor_imp): Add
	static assertions to enforce requirements on pointer types.
	(__resource_adaptor_imp::get_allocator()): Add noexcept.
	(new_delete_resource, null_memory_resource): Return address of an
	object with dynamic storage duration.
	(__null_memory_resource): Remove.
	* testsuite/experimental/memory_resource/70966.cc: New.

From-SVN: r261818