A recently approved change for the C++23 working draft.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (__cpp_lib_string_resize_and_overwrite):
	Define for C++23.
	(basic_string::resize_and_overwrite): Declare.
	* include/bits/basic_string.tcc (basic_string::resize_and_overwrite):
	Define.
	* include/std/version (__cpp_lib_resize_and_overwrite): Define
	for C++23.
	* testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc:
	New test.

Tested powerpc64le-linux. Committed to trunk.