libstdc++: Work around clang misdesign in time_get<>::get [PR104990]

libstdc++: Work around clang misdesign in time_get<>::get [PR104990]

Apparently clang has a -fgnuc-version= option which allows it to pretend
it is any GCC version the user likes.  It is already bad that it claims to
be GCC 4.2 compatible by default when it is not (various unimplemented
extensions at least), but this option is a horrible idea.

Anyway, this patch adds a hack for it.

2022-03-21  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/104990
	* include/bits/locale_facets_nonio.tcc (get): Don't check if do_get
	isn't overloaded if __clang__ is defined.