[libc++][spaceship] P1614R2: Added `operator==` to `file_status`

Implemented parts of P1614R2:
- Added `operator==` to `file_status`

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D152647

GitOrigin-RevId: a9e0321ffed19cb74b5b238af0c155bab1b4093a
4 files changed