Add getDeserializationListener to ASTReader

[yamaguchi]

Summary

We need to know if ASTReader already has a DeserializationListner or
not, and this also helps to create a multiplexing deserialization
listener if there is one already attached.

[rsmith]

This seems fine, but I wonder if we should expose a mechanism that will create a multiplex listener automatically as needed?

[yamaguchi]

Do you mean something like addDeserializationListener which create a multiplex listener with an existing listener? I think that'll be good as well!

clang/include/clang/Serialization/ASTReader.h

~ListenerScope() {
                                ->takeSecond());
     }
   };
 
   /// \brief Set the AST deserialization listener.
   void setDeserializationListener(ASTDeserializationListener *Listener,
                                   bool TakeOwnership = false);
 
+  /// \brief Get the AST deserialization listener.
+  ASTDeserializationListener *getDeserializationListener() {
+    return DeserializationListener;
+  }
+
   /// \brief Determine whether this AST reader has a global index.
   bool hasGlobalIndex() const { return (bool)GlobalIndex; }
 
   /// \brief Return global module index.
   GlobalModuleIndex *getGlobalIndex() { return GlobalIndex.get(); }
 
   /// \brief Reset reader for a reload try.
   void resetForReload() { TriedLoadingGlobalIndex = false; }