Basic: import IntrusiveRefCntPtr<> into clang namespace

The class name is long enough without the llvm:: added.
Also bring in RefCountedBase and RefCountedBaseVPTR.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@150958 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/Warnings.cpp b/lib/Frontend/Warnings.cpp
index 1866949..ec5fde0 100644
--- a/lib/Frontend/Warnings.cpp
+++ b/lib/Frontend/Warnings.cpp
@@ -73,7 +73,7 @@
     Diags.setExtensionHandlingBehavior(DiagnosticsEngine::Ext_Ignore);
 
   llvm::SmallVector<diag::kind, 10> _Diags;
-  const llvm::IntrusiveRefCntPtr< DiagnosticIDs > DiagIDs =
+  const IntrusiveRefCntPtr< DiagnosticIDs > DiagIDs =
     Diags.getDiagnosticIDs();
   // We parse the warning options twice.  The first pass sets diagnostic state,
   // while the second pass reports warnings/errors.  This has the effect that