[clang] Compare constraints before diagnosing mismatched ref qualifiers (GH58962)

As noticed in GH58962, we should only diagnose illegal overloads of member functions
when the ref qualifiers don't match if the trailing constraints are the same.

The fix is to move the existing constraints check earlier in Sema::IsOverload.

Closes https://github.com/llvm/llvm-project/issues/58962

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D138749