[orc] Add support for LLVM RTTI to LLJIT and LLLayzJIT

[lxsameer]

Summary

With this patch it would be possible to use llvm's RTTI utilities with LLJIT and LLLazyJIT and dynamically cast a LLJIT pointer (that points to a LLLazyJIT) to a LLLazyJIT pointer.

llvm/include/llvm/ExecutionEngine/Orc/LLJIT.h

public:
 
     virtual Error deinitialize(JITDylib &JD) = 0;
 
   protected:
     static void setInitTransform(LLJIT &J,
                                  IRTransformLayer::TransformFunction T);
   };
 
+  enum JITKind { JK_LLJIT = 0, JK_LLLAZYJIT };
+
+  virtual JITKind getKind() const { return JK_LLJIT; }
+
+  static bool classof(const LLJIT *JIT) { return JIT->getKind() == JK_LLJIT; }
+
   /// Destruct this instance. If a multi-threaded instance, waits for all
   /// compile threads to complete.
   virtual ~LLJIT();
 
   /// Returns the ExecutionSession for this instance.
   ExecutionSession &getExecutionSession() { return *ES; }
 
   /// Returns a reference to the triple for this instance.
public:
   /// Add a module to be lazily compiled to JITDylib JD.
   Error addLazyIRModule(JITDylib &JD, ThreadSafeModule M);
 
   /// Add a module to be lazily compiled to the main JITDylib.
   Error addLazyIRModule(ThreadSafeModule M) {
     return addLazyIRModule(*Main, std::move(M));
   }
 
-private:
+  JITKind getKind() const override { return JK_LLLAZYJIT; }
 
+  static bool classof(const LLJIT *JIT) {
+    return JIT->getKind() == JK_LLLAZYJIT;
+  }
+
+private:
   // Create a single-threaded LLLazyJIT instance.
   LLLazyJIT(LLLazyJITBuilderState &S, Error &Err);
 
   std::unique_ptr<LazyCallThroughManager> LCTMgr;
   std::unique_ptr<CompileOnDemandLayer> CODLayer;
 };
 
 class LLJITBuilderState {