libstdc++: Redefine __glibcxx_assert to work in C++23 constexpr

libstdc++: Redefine __glibcxx_assert to work in C++23 constexpr

The changes in r14-5979 to support unknown references in constant
expressions caused some test regressions. The way that __glibcxx_assert
is defined for constant evaluation no longer works when
_GLIBCXX_ASSERTIONS is defined.

This change simplifies __glibcxx_assert so that there is only one check,
rather than a constexpr one and a conditionally-enabled runtime one. The
constexpr one does not need to use __builtin_unreachable to cause a
compilation failure, because __glibcxx_assert_fail is not usable in
constant expressions, so that will cause a failure too.

As well as fixing the regressions, this makes the code for the
assertions shorter and simpler, so should be quicker to compile, and
might inline better too.

libstdc++-v3/ChangeLog:

	* include/bits/c++config (__glibcxx_assert_fail): Declare even
	when assertions are not enabled.
	(__glibcxx_constexpr_assert): Remove macro.
	(__glibcxx_assert_impl): Remove macro.
	(_GLIBCXX_ASSERT_FAIL): New macro.
	(_GLIBCXX_DO_ASSERT): New macro.
	(__glibcxx_assert): Simplify to a single definition that works
	at runtime and during constant evaluation.
	* testsuite/21_strings/basic_string_view/element_access/char/back_constexpr_neg.cc:
	Adjust expected errors.
	* testsuite/21_strings/basic_string_view/element_access/char/constexpr_neg.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/element_access/char/front_constexpr_neg.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/element_access/wchar_t/back_constexpr_neg.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/element_access/wchar_t/constexpr_neg.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/element_access/wchar_t/front_constexpr_neg.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/modifiers/remove_prefix/debug.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/modifiers/remove_suffix/debug.cc:
	Likewise.
	* testsuite/23_containers/span/back_neg.cc: Likewise.
	* testsuite/23_containers/span/front_neg.cc: Likewise.
	* testsuite/23_containers/span/index_op_neg.cc: Likewise.
	* testsuite/26_numerics/lcm/105844.cc: Likewise.