getArchTypeForDarwinArchName is only used in the clang driver, copy it there.
I will remove it from llvm in the next commit.

llvm-svn: 167156
diff --git a/clang/lib/Driver/Tools.h b/clang/lib/Driver/Tools.h
index 9d0778b..5898c66 100644
--- a/clang/lib/Driver/Tools.h
+++ b/clang/lib/Driver/Tools.h
@@ -202,6 +202,8 @@
 
 
 namespace darwin {
+  llvm::Triple::ArchType getArchTypeForDarwinArchName(StringRef Str);
+
   class LLVM_LIBRARY_VISIBILITY DarwinTool : public Tool {
     virtual void anchor();
   protected: