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

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

Since GCC 6.1 there is no reason we can't just use __glibcxx_assert in
constexpr functions in string_view. 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.

(cherry picked from commit 3eefb302d2bd8502cb3d8fe44e672b11092ccaf6)