Fix -Wsign-compare warnings reported by clang++.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@98174 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Driver/ArgList.cpp b/lib/Driver/ArgList.cpp
index 8a57d14..07d24c9 100644
--- a/lib/Driver/ArgList.cpp
+++ b/lib/Driver/ArgList.cpp
@@ -80,9 +80,9 @@
   Arg *A1 = getLastArgNoClaim(Id1);
   Arg *A2 = getLastArgNoClaim(Id2);
 
-  int A0Idx = A0 ? A0->getIndex() : -1;
-  int A1Idx = A1 ? A1->getIndex() : -1;
-  int A2Idx = A2 ? A2->getIndex() : -1;
+  int A0Idx = A0 ? (int) A0->getIndex() : -1;
+  int A1Idx = A1 ? (int) A1->getIndex() : -1;
+  int A2Idx = A2 ? (int) A2->getIndex() : -1;
 
   if (A0Idx > A1Idx) {
     if (A0Idx > A2Idx)