libstdc++: Fix _GLIBCXX_DEBUG mode constexpr compatibility

libstdc++: Fix _GLIBCXX_DEBUG mode constexpr compatibility

The __glibcxx_check_can_[increment|decrement]_range macros are using the
_GLIBCXX_DEBUG_VERIFY_COND_AT macro which is not constexpr compliant and will produce nasty
diagnostics rather than the std::__failed_assertion dedicated to constexpr. Replace it with
correct _GLIBCXX_DEBUG_VERIFY_AT_F.

libstdc++-v3/ChangeLog:

	* include/debug/macros.h (__glibcxx_check_can_increment_range): Replace
	_GLIBCXX_DEBUG_VERIFY_COND_AT usage with _GLIBCXX_DEBUG_VERIFY_AT_F.
	(__glibcxx_check_can_decrement_range): Likewise.
	* testsuite/25_algorithms/copy_backward/constexpr.cc (test03): New.
	* testsuite/25_algorithms/copy/debug/constexpr_neg.cc: New test.
	* testsuite/25_algorithms/copy_backward/debug/constexpr_neg.cc: New test.
	* testsuite/25_algorithms/equal/constexpr_neg.cc: New test.
	* testsuite/25_algorithms/equal/debug/constexpr_neg.cc: New test.