[JITLink][ORC] Make the LinkGraph available to modifyPassConfig.

This makes the target triple, graph name, and full graph content available
when making decisions about how to populate the linker pass pipeline.

Also updates the LLJITWithObjectLinkingLayerPlugin example to show more
API use, including use of the API changes in this patch.

lib/ExecutionEngine/Orc/DebugObjectManagerPlugin.cpp

@@ -432,7 +432,7 @@ void DebugObjectManagerPlugin::notifyMaterializing(
 }
 
 void DebugObjectManagerPlugin::modifyPassConfig(
-    MaterializationResponsibility &MR, const Triple &TT,
+    MaterializationResponsibility &MR, LinkGraph &G,
     PassConfiguration &PassConfig) {
   // Not all link artifacts have associated debug objects.
   std::lock_guard<std::mutex> Lock(PendingObjsLock);