Clang driver changes for iOS 5.0 and OS X Lion support.

Check whether the libc++ library is available when using -stdlib=libc++,
and also adjust the check for whether to link with -lgcc_s.1.
Patch by Ted Kremenek and Daniel Dunbar.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@141374 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Driver/ToolChains.h b/lib/Driver/ToolChains.h
index c9ff3a0..dfcb253 100644
--- a/lib/Driver/ToolChains.h
+++ b/lib/Driver/ToolChains.h
@@ -66,6 +66,12 @@
     ARCSimulator_NoARCRuntime
   } ARCRuntimeForSimulator;
 
+  mutable enum {
+    LibCXXSimulator_None,
+    LibCXXSimulator_NotAvailable,
+    LibCXXSimulator_Available
+  } LibCXXForSimulator;
+
 private:
   /// Whether we are targeting iPhoneOS target.
   mutable bool TargetIsIPhoneOS;