[Clang] Reject in-class defaulting of previously declared comparison operators

Comparison operators are not allowed to be defaulted if they were previously declared outside the class.
Pretty low-impact, but it's nice to reject this without a linking error.
Fixes https://github.com/llvm/llvm-project/issues/51227.

Reviewed By: #clang-language-wg, ChuanqiXu

Differential Revision: https://reviews.llvm.org/D141803