libstdc++: Fix effects of combining locales [PR108323]

libstdc++: Fix effects of combining locales [PR108323]

This fixes a bug in locale::combine where we fail to meet the standard's
requirement that the result is unnamed. It also implements two library
issues related to the names of combined locales (2295 and 3676).

libstdc++-v3/ChangeLog:

	PR libstdc++/108323
	* include/bits/locale_classes.tcc (locale(const locale&, Facet*)):
	Return a copy of the first argument when the facet pointer is
	null, as per LWG 2295.
	(locale::combine): Ensure the result is unnamed.
	* src/c++11/localename.cc (_M_replace_categories): Ignore
	whether the second locale has a name when cat == none, as per
	LWG 3676.
	* src/c++98/locale.cc (_M_install_facet): Use __builtin_expect
	to predict that the facet pointer is non-null.
	* testsuite/22_locale/locale/cons/names.cc: New test.