libstdc++: Use std::type_identity_t in <string_view> as per LWG 3950 [PR114400]

The difference between __type_identity_t and std::type_identity_t is
observable, as demonstrated in the PR. Nobody in LWG seems to think this
an example we should really care about, but it seems easy and harmless
to change this.

libstdc++-v3/ChangeLog:

	PR libstdc++/114400
	* include/std/string_view (operator==): Use std::type_identity_t
	in C++20 instead of our own __type_identity_t.