[LLJIT] Add API to expose linking layer from LLJIT classes

[beanz]

Summary

This patch just surfaces the object linking layer from the LLJIT classes so that clients can take advantage of the changes implemented in r341154.

[lhames]

LGTM. Thanks Chris!

include/llvm/ExecutionEngine/Orc/LLJIT.h

public:
   }
 
   /// Runs all not-yet-run static constructors.
   Error runConstructors() { return CtorRunner.run(); }
 
   /// Runs all not-yet-run static destructors.
   Error runDestructors() { return DtorRunner.run(); }
 
+  /// Returns a reference to the ObjLinkingLayer
+  RTDyldObjectLinkingLayer2 &getObjLinkingLayer() { return ObjLinkingLayer; }
+
 protected:
   LLJIT(std::unique_ptr<ExecutionSession> ES, std::unique_ptr<TargetMachine> TM,
         DataLayout DL);
 
   std::shared_ptr<RuntimeDyld::MemoryManager> getMemoryManager(VModuleKey K);
 
   std::string mangle(StringRef UnmangledName);