libstdc++: Fix up constexpr std::char_traits<char>::compare [PR99181]

Because of LWG 467, std::char_traits<char>::lt compares the values
cast to unsigned char rather than char, so even when char is signed
we get unsigned comparision.  std::char_traits<char>::compare uses
__builtin_memcmp and that works the same, but during constexpr evaluation
we were calling __gnu_cxx::char_traits<char_type>::compare.  As
char_traits::lt is not virtual, __gnu_cxx::char_traits<char_type>::compare
used __gnu_cxx::char_traits<char_type>::lt rather than
std::char_traits<char>::lt and thus compared chars as signed if char is
signed.
This change fixes it by inlining __gnu_cxx::char_traits<char_type>::compare
into std::char_traits<char>::compare by hand, so that it calls the right
lt method.

2021-02-23  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/99181
	* include/bits/char_traits.h (char_traits<char>::compare): For
	constexpr evaluation don't call
	__gnu_cxx::char_traits<char_type>::compare but do the comparison loop
	directly.

	* testsuite/21_strings/char_traits/requirements/char/99181.cc: New
	test.

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