libstdc++: Enable assertions in constexpr string_view members [PR 71960]

libstdc++: Enable assertions in constexpr string_view members [PR 71960]

There is no longer any reason we can't just use __glibcxx_assert in
constexpr functions. As long as the condition is true, there will be no
call to std::__replacement_assert that would make the function
ineligible for constant evaluation.

	PR libstdc++/71960
	* include/experimental/string_view (basic_string_view):
	Enable debug assertions.
	* include/std/string_view (basic_string_view):
	Likewise.