libstdc++: diagnose some misuses of [locale.convenience] functions [PR 89728]

This patch provides better diagnostics for common misuse of
[locale.convenience] functions with std::string as a character type.

libstdc++-v3/ChangeLog:

	PR libstdc++/89728
	* include/bits/locale_facets.h (ctype<basic_string<C,T,A>>):
	Declare (but do not define) partial specialization.
	* testsuite/22_locale/ctype/is/string/89728_neg.cc: New test.