fix lookup of bits/c++config.h on mingw, patch by Ismail Donmez!

llvm-svn: 112969
diff --git a/clang/lib/Frontend/InitHeaderSearch.cpp b/clang/lib/Frontend/InitHeaderSearch.cpp
index 3af2adb..df91713 100644
--- a/clang/lib/Frontend/InitHeaderSearch.cpp
+++ b/clang/lib/Frontend/InitHeaderSearch.cpp
@@ -195,6 +195,8 @@
           System, true, false, false);
   AddPath(Base + "/" + Arch + "/" + Version + "/include/c++",
           System, true, false, false);
+  AddPath(Base + "/" + Arch + "/" + Version + "/include/c++/" + Arch,
+          System, true, false, false);
   AddPath(Base + "/" + Arch + "/" + Version + "/include/c++/backward",
           System, true, false, false);
 }