[clang][c++20][NFC] ComparisonCategoryType - updated comment, remov…

…ed `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

clang/include/clang/AST/ComparisonCategories.h

@@ -39,9 +39,8 @@ class NamespaceDecl;
 /// An enumeration representing the different comparison categories
 /// types.
 ///
-/// C++2a [cmp.categories.pre] The types weak_equality, strong_equality,
-/// partial_ordering, weak_ordering, and strong_ordering are collectively
-/// termed the comparison category types.
+/// C++20 [cmp.categories.pre] The types partial_ordering, weak_ordering, and
+/// strong_ordering are collectively termed the comparison category types.
 enum class ComparisonCategoryType : unsigned char {
   PartialOrdering,
   WeakOrdering,