[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.

Author: DamonFool

clang/lib/Sema/SemaDecl.cpp

@@ -11650,7 +11650,7 @@ static bool CheckMultiVersionAdditionalDecl(
   const auto *NewTA = NewFD->getAttr<TargetAttr>();
   const auto *NewTVA = NewFD->getAttr<TargetVersionAttr>();
   MultiVersionKind NewMVKind = NewFD->getMultiVersionKind();
-  MultiVersionKind OldMVKind = OldFD->getMultiVersionKind();
+  [[maybe_unused]] MultiVersionKind OldMVKind = OldFD->getMultiVersionKind();
 
   ParsedTargetAttr NewParsed;
   if (NewTA) {