[libc++] Fixes string_view comparison operators.

While implementing `operator<=>` for `string_view` (D130295) @philnik
pointed out `common_type` should be `type_identity`. Since it was an
existing issue that wasn't addressed.

This addresses the issue for both the new and existing equality and
comparison operators. The test is based on the example posted in
D130295.

Reviewed By: philnik, #libc, huixie90

Differential Revision: https://reviews.llvm.org/D131322