[clang] Fix -Wunused-variable in SemaDecl.cpp (NFC)

llvm-project/clang/lib/Sema/SemaDecl.cpp:11653:20:
error: unused variable 'OldMVKind' [-Werror,-Wunused-variable]
  MultiVersionKind OldMVKind = OldFD->getMultiVersionKind();
                   ^
1 error generated.