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

llvm-project/clang/lib/Sema/SemaCast.cpp:503:23:
error: unused variable 'Res' [-Werror,-Wunused-variable]
    OverloadingResult Res =
                      ^
1 error generated.

Author: DamonFool

clang/lib/Sema/SemaCast.cpp

@@ -500,7 +500,7 @@ static bool tryDiagnoseOverloadedCast(Sema &S, CastType CT,
 
   case OR_Deleted: {
     OverloadCandidateSet::iterator Best;
-    OverloadingResult Res =
+    [[maybe_unused]] OverloadingResult Res =
         candidates.BestViableFunction(S, range.getBegin(), Best);
     assert(Res == OR_Deleted && "Inconsistent overload resolution");