libstdc++: Enable constexpr std::bitset for debug mode

libstdc++: Enable constexpr std::bitset for debug mode

We already disable all debug mode checks for C++11 and later, so we can
easily make everything constexpr. This fixes the FAIL results for the
new tests when using -D_GLIBCXX_DEBUG.

Also fix some other tests failing with non-default test flags.

libstdc++-v3/ChangeLog:

	* include/debug/bitset (__debug::bitset): Add constexpr to all
	member functions.
	(operator&, operator|, operator^): Add inline and constexpr.
	(operator>>, operator<<): Add inline.
	* testsuite/20_util/bitset/access/constexpr.cc: Only check using
	constexpr std::string for the cxx11 ABI.
	* testsuite/20_util/bitset/cons/constexpr_c++23.cc: Likewise.
	* testsuite/20_util/headers/bitset/synopsis.cc: Check constexpr
	for C++23.