libstdc++: Disable __gnu_debug::__is_singular(T*) in constexpr [PR109517]

libstdc++: Disable __gnu_debug::__is_singular(T*) in constexpr [PR109517]

Because of PR c++/85944 we have several bugs where _GLIBCXX_DEBUG causes
errors for constexpr code. Although Bug 117966 could be fixed by
avoiding redundant debug checks in std::span, and Bug 106212 could be
fixed by avoiding redundant debug checks in std::array, there are many
more cases where similar __glibcxx_requires_valid_range checks fail to
compile and removing the checks everywhere isn't desirable.

This just disables the __gnu_debug::__check_singular(T*) check during
constant evaluation. Attempting to dereference a null pointer will
certainly fail during constant evaluation (if it doesn't fail then it's
a compiler bug and not the library's problem). Disabling this check
during constant evaluation shouldn't do any harm.

libstdc++-v3/ChangeLog:

	PR libstdc++/109517
	PR libstdc++/109976
	* include/debug/helper_functions.h (__valid_range_aux): Treat
	all input iterator ranges as valid during constant evaluation.