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

[H-G-Hristov]

Summary

weak_equality and strong_equality are removed from the standard:

• https://wg21.link/P1959R0
• https://eel.is/c++draft/cmp.categories.pre

clang/include/clang/AST/ComparisonCategories.h

 class CXXRecordDecl;
 class Sema;
 class QualType;
 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,
   StrongOrdering,
   First = PartialOrdering,
   Last = StrongOrdering
 };