[ORC] Make LLJIT's destructor virtual.

This allows LLLazyJIT instances to be safely owned by unique_ptr<LLJIT>s.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D119064

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

@@ -56,7 +56,7 @@ public:
 
   /// Destruct this instance. If a multi-threaded instance, waits for all
   /// compile threads to complete.
-  ~LLJIT();
+  virtual ~LLJIT();
 
   /// Returns the ExecutionSession for this instance.
   ExecutionSession &getExecutionSession() { return *ES; }