[libc++] [P0887] Add newest feature-test macros; mark `type_identity` as implemented.

`__cpp_lib_type_identity` was implemented way back in cf49ccd0 (Clang 8),
probably before the feature-test macro had been settled on.

`__cpp_lib_string_resize_and_overwrite` will be added by D113013 so I didn't add it here.

Fixes #46605.

Differential Revision: https://reviews.llvm.org/D116433