Handle replacement into a position past the original member.

We were incorrectly computing where to insert a member if it was replacing
a previous member that was before the insert point.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186792 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/llvm-ar/llvm-ar.cpp b/tools/llvm-ar/llvm-ar.cpp
index 4ade562..b1c8986 100644
--- a/tools/llvm-ar/llvm-ar.cpp
+++ b/tools/llvm-ar/llvm-ar.cpp
@@ -458,10 +458,10 @@
   int InsertPos = -1;
   StringRef PosName = sys::path::filename(RelPos);
   if (OldArchive) {
-    int Pos = 0;
     for (object::Archive::child_iterator I = OldArchive->begin_children(),
                                          E = OldArchive->end_children();
-         I != E; ++I, ++Pos) {
+         I != E; ++I) {
+      int Pos = Ret.size();
       StringRef Name;
       failIfError(I->getName(Name));
       if (Name == PosName) {