libstdc++: Check string value_type in std::make_format_args [PR112607]

libstdc++-v3/ChangeLog:

	PR libstdc++/112607
	* include/std/format (basic_format_arg::_S_to_arg_type): Check
	value_type for basic_string_view and basic_string
	specializations.
	* testsuite/std/format/arguments/112607.cc: New test.

(cherry picked from commit 279e407a)