libstdc++: Poor man's case insensitive comparisons in time_get [PR71557]

This patch uses the same not completely correct case insensitive comparisons
as used elsewhere in the same header.  Proper comparisons that would handle
even multi-byte characters would be harder, but I don't see them implemented
in __ctype's methods.

2021-12-15  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/71557
	* include/bits/locale_facets_nonio.tcc (_M_extract_via_format):
	Compare characters other than format specifiers and whitespace
	case insensitively.
	(_M_extract_name): Compare characters case insensitively.
	* testsuite/22_locale/time_get/get/char/71557.cc: New test.
	* testsuite/22_locale/time_get/get/wchar_t/71557.cc: New test.