[clang] Fix diagnostics for defaulted, implicitly deleted 'operator=='.

[clang] Fix diagnostics for defaulted, implicitly deleted 'operator=='.

https://godbolt.org/z/cMKE3o1aG

According to the issue https://github.com/llvm/llvm-project/issues/63960 , compiler falsely complains that no viable `operator<=>` was found while we are actually looking for a `operator==`.
This bug has been fixed through adding a check of the `OverloadedOperatorKind` type's object, `OO`, to see if it is `OO_EqualEqual` in addition to `OO_ExclaimEqual`.

Reviewed By: #clang-language-wg, cor3ntin

Differential Revision: https://reviews.llvm.org/D155714