[clang][Sema] Fix typo in checkBuiltinArgument helper

The checkBuiltinArgument helper takes an integer ArgIndex and is
documented as performing normal type-checking on that argument. However,
it mistakenly hardcodes the argument index to zero when retrieving the
argument from the call expression.

This hadn't been noticed previously as all in-tree uses typecheck the
0th argument anyway.

clang/lib/Sema/SemaChecking.cpp

@@ -6222,7 +6222,7 @@ static bool checkBuiltinArgument(Sema &S, CallExpr *E, unsigned ArgIndex) {
   InitializedEntity Entity =
     InitializedEntity::InitializeParameter(S.Context, Param);
 
-  ExprResult Arg = E->getArg(0);
+  ExprResult Arg = E->getArg(ArgIndex);
   Arg = S.PerformCopyInitialization(Entity, SourceLocation(), Arg);
   if (Arg.isInvalid())
     return true;