[Frontend] Fix an issue where a quoted search path is incorrectly
removed as a duplicate header search path

The commit r126167 started passing the First index into RemoveDuplicates, but
forgot to update 0 to First in the loop that looks for the duplicate. This
resulted in a bug where an -iquoted search path was incorrectly removed if you
passed in the same path into -iquote and more than one time into -isystem.

rdar://23991350

Differential Revision: https://reviews.llvm.org/D27298

llvm-svn: 288491
diff --git a/clang/lib/Frontend/InitHeaderSearch.cpp b/clang/lib/Frontend/InitHeaderSearch.cpp
index c76ccea..d50fb6d 100644
--- a/clang/lib/Frontend/InitHeaderSearch.cpp
+++ b/clang/lib/Frontend/InitHeaderSearch.cpp
@@ -526,7 +526,7 @@
     if (CurEntry.getDirCharacteristic() != SrcMgr::C_User) {
       // Find the dir that this is the same of.
       unsigned FirstDir;
-      for (FirstDir = 0; ; ++FirstDir) {
+      for (FirstDir = First;; ++FirstDir) {
         assert(FirstDir != i && "Didn't find dupe?");
 
         const DirectoryLookup &SearchEntry = SearchList[FirstDir];