libstdc++: Fix non-reserved name in std::allocator base class [PR64135]

The possible base classes of std::allocator are new_allocator and
malloc_allocator, which both cause a non-reserved name to be declared in
every program that includes the definition of std::allocator. This is
non-conforming.

This change replaces __gnu_cxx::new_allocator with std::__new_allocator
which is identical except for using a reserved name. The non-standard
extension __gnu_cxx::new_allocator is preserved as a thin wrapper over
std::__new_allocator. There is no problem with the extension using a
non-reserved name now that it's not included by default in other
headers.

The same change could be done to __gnu_cxx::malloc_allocator but as it's
not the default configuration it can wait.

libstdc++-v3/ChangeLog:

	PR libstdc++/64135
	* config/allocator/new_allocator_base.h: Include
	<bits/new_allocator.h> instead of <ext/new_allocator.h>.
	(__allocator_base): Use std::__new_allocator instead of
	__gnu_cxx::new_allocator.
	* doc/xml/manual/allocator.xml: Document new default base class
	for std::allocator.
	* doc/xml/manual/evolution.xml: Likewise.
	* doc/html/*: Regenerate.
	* include/Makefile.am: Add bits/new_allocator.h.
	* include/Makefile.in: Regenerate.
	* include/experimental/memory_resource (new_delete_resource):
	Use std::__new_allocator instead of __gnu_cxx::new_allocator.
	* include/ext/new_allocator.h (new_allocator): Derive from
	std::__new_allocator. Move implementation to ...
	* include/bits/new_allocator.h: New file.
	* testsuite/20_util/allocator/64135.cc: New test.