libstdc++: Make _GLIBCXX_DEBUG checks constexpr compatible

libstdc++: Make _GLIBCXX_DEBUG checks constexpr compatible

libstdc++-v3/ChangeLog:

	* include/debug/assertions.h (__glibcxx_requires_non_empty_range):
	Remove __builtin_expect.
	(__glibcxx_requires_subscript): Likewise.
	(__glibcxx_requires_nonempty): Likewise.
	* include/debug/formatter.h (__check_singular): Add C++11 constexpr
	qualification.
	* include/debug/helper_functions.h (__check_singular): Likewise. Skip
	check if constant evaluated.
	(__valid_range): Do not skip check if constant evaluated.
	* include/debug/macros.h (_GLIBCXX_DEBUG_VERIFY_COND_AT): Add
	__builtin_expect.
	(_GLIBCXX_DEBUG_VERIFY_AT_F): Use __glibcxx_assert_1.
	* testsuite/21_strings/basic_string_view/element_access/char/back_constexpr_neg.cc:
	New test.
	* testsuite/21_strings/basic_string_view/element_access/char/constexpr.cc: New test.
	* testsuite/21_strings/basic_string_view/element_access/char/constexpr_neg.cc: New test.
	* testsuite/21_strings/basic_string_view/element_access/char/front_back_constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string_view/element_access/char/front_constexpr_neg.cc:
	New test.
	* testsuite/21_strings/basic_string_view/element_access/wchar_t/back_constexpr_neg.cc:
	New test.
	* testsuite/21_strings/basic_string_view/element_access/wchar_t/constexpr.cc: New test.
	* testsuite/21_strings/basic_string_view/element_access/wchar_t/constexpr_neg.cc: New test.
	* testsuite/21_strings/basic_string_view/element_access/wchar_t/front_constexpr_neg.cc:
	New test.
	* testsuite/25_algorithms/lower_bound/debug/constexpr_partitioned_neg.cc: New test.
	* testsuite/25_algorithms/lower_bound/debug/constexpr_partitioned_pred_neg.cc: New test.
	* testsuite/25_algorithms/lower_bound/debug/constexpr_valid_range_neg.cc: New test.
	* testsuite/25_algorithms/lower_bound/debug/partitioned_neg.cc: New test.
	* testsuite/25_algorithms/lower_bound/debug/partitioned_pred_neg.cc: New test.
	* testsuite/25_algorithms/upper_bound/debug/constexpr_partitioned_neg.cc: New test.
	* testsuite/25_algorithms/upper_bound/debug/constexpr_partitioned_pred_neg.cc: New test.
	* testsuite/25_algorithms/upper_bound/debug/constexpr_valid_range_neg.cc: New test.
	* testsuite/25_algorithms/upper_bound/debug/partitioned_neg.cc: New test.
	* testsuite/25_algorithms/upper_bound/debug/partitioned_pred_neg.cc: New test.