libstdc++: Use if-constexpr in std::__try_use_facet [PR113099]

As noted in the PR, we can use if-constexpr for the explicit
instantantiation definitions that are compiled with -std=gnu++11. We
just need to disable the -Wc++17-extensions diagnostics.

libstdc++-v3/ChangeLog:

	PR libstdc++/113099
	* include/bits/locale_classes.tcc (__try_use_facet): Use
	if-constexpr for C++11 and up.