libstdc++: Implement std::unreachable() for C++23 (P0627R6)

This defines std::unreachable as an assertion for debug mode, a trap
when _GLIBCXX_ASSERTIONS is defined, and __builtin_unreachable()
otherwise.

The reason for only using __builtin_trap() in the second case is to
avoid the overhead of setting up a call to __glibcxx_assert_fail that
should never happen.

UBsan can detect if __builtin_unreachable() is executed, so if a feature
test macro for that sanitizer is added, we could change just use
__builtin_unreachable() when the sanitizer is enabled.

While thinking about what the debug assertion failure should print, I
noticed that the __glibcxx_assert_fail function doesn't check for null
pointers. This adds a check so we don't try to print them if null.

libstdc++-v3/ChangeLog:

	* include/std/utility (unreachable): Define for C++23.
	* include/std/version (__cpp_lib_unreachable): Define.
	* src/c++11/debug.cc (__glibcxx_assert_fail): Check for valid
	arguments. Handle only the function being given.
	* testsuite/20_util/unreachable/1.cc: New test.
	* testsuite/20_util/unreachable/version.cc: New test.

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