libstdc++: Backport std::basic_string::_S_allocate from trunk

This is a backport of r14-739-gc62e945492afbb to keep the exported
symbol list consistent between trunk and gcc-13. The new assertions from
that commit are not part of this backport.

libstdc++-v3/ChangeLog:

	* config/abi/pre/gnu.ver: Export basic_string::_S_allocate.
	* include/bits/basic_string.h: (basic_string::_Alloc_traits_impl):
	Remove class template.
	(basic_string::_S_allocate): New static member function.
	(basic_string::assign): Use _S_allocate.
	* include/bits/basic_string.tcc (basic_string::_M_create)
	(basic_string::reserve, basic_string::_M_replace): Likewise.

(cherry picked from commit c62e9454)