libstdc++: Fix recent changes to __glibcxx_assert [PR112882]

libstdc++: Fix recent changes to __glibcxx_assert [PR112882]

The changes in r14-6198-g5e8a30d8b8f4d7 were broken, as I used
_GLIBCXX17_CONSTEXPR for the 'if _GLIBCXX17_CONSTEXPR (true)' condition,
forgetting that it would also be used for the is_constant_evaluated()
check. Using 'if constexpr (std::is_constant_evaluated())' is a bug.

Additionally, relying on __glibcxx_assert_fail to give a "not a constant
expression" error is a problem because at -O0 an undefined reference to
__glibcxx_assert_fail is present in the compiled code. This means you
can't use libstdc++ headers without also linking to libstdc++ for the
symbol definition.

This fix rewrites the __glibcxx_assert macro again. This still avoids
doing the duplicate checks, once for constexpr and once at runtime (if
_GLIBCXX_ASSERTIONS is defined). When _GLIBCXX_ASSERTIONS is defined we
still rely on __glibcxx_assert_fail to give a "not a constant
expression" error during constant evaluation (because when assertions
are defined it's not a problem to emit a reference to the symbol). But
when that macro is not defined, we use a new inline (but not constexpr)
overload of __glibcxx_assert_fail to cause compilation to fail. That
inline function doesn't cause an undefined reference to a symbol in the
library (and will be optimized away anyway).

We can also add always_inline to the __is_constant_evaluated function,
although this doesn't actually matter for -O0 and it's always inlined
with any optimization enabled.

libstdc++-v3/ChangeLog:

	PR libstdc++/112882
	* include/bits/c++config (__is_constant_evaluated): Add
	always_inline attribute.
	(_GLIBCXX_DO_ASSERT): Remove macro.
	(__glibcxx_assert): Define separately for assertions-enabled and
	constexpr-only cases.