[clang][c++20][NFC] `ComparisonCategoryType` - updated comment, removed `std:*_equality`

`weak_equality` and `strong_equality` are removed from the standard:
- https://wg21.link/P1959R0
- https://eel.is/c++draft/cmp.categories.pre

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D151627