[clang][sema] Ignore xor-used-as-pow if both sides are macros

This happens in codebases a lot, which use xor where both sides are
macros. Using xor in that case is not the common error-prone 2^6 code
that the warning was introduced for.

Don't diagnose such a use of xor.

Differential Revision: https://reviews.llvm.org/D97445