[ORC] Set setCloneToNewContextOnEmit on LLJIT's transform layer when needed.

Based on Don Hinton's patch in https://reviews.llvm.org/D72406. This feature
was accidentally left out of e9e26c0, and
would have pessimized concurrent compilation in the default case.

Thanks for spotting this Don!

Author: lhames

llvm/lib/ExecutionEngine/Orc/LLJIT.cpp

@@ -150,10 +150,11 @@ LLJIT::LLJIT(LLJITBuilderState &S, Error &Err)
     }
     CompileLayer = std::make_unique<IRCompileLayer>(
         *ES, ObjTransformLayer, std::move(*CompileFunction));
+    TransformLayer = std::make_unique<IRTransformLayer>(*ES, *CompileLayer);
   }
 
   if (S.NumCompileThreads > 0) {
-    CompileLayer->setCloneToNewContextOnEmit(true);
+    TransformLayer->setCloneToNewContextOnEmit(true);
     CompileThreads = std::make_unique<ThreadPool>(S.NumCompileThreads);
     ES->setDispatchMaterialization(
         [this](JITDylib &JD, std::unique_ptr<MaterializationUnit> MU) {
@@ -163,8 +164,6 @@ LLJIT::LLJIT(LLJITBuilderState &S, Error &Err)
           CompileThreads->async(std::move(Work));
         });
   }
-
-  TransformLayer = std::make_unique<IRTransformLayer>(*ES, *CompileLayer);
 }
 
 std::string LLJIT::mangle(StringRef UnmangledName) {