[ORC] Use ExecutionSession's pre-constructed main JITDylib in LLJIT.

As of r342086 ExecutionSession automatically creates a 'main' JITDylib, so
there is no need for LLJIT to create its own.

llvm-svn: 343167
diff --git a/llvm/lib/ExecutionEngine/Orc/LLJIT.cpp b/llvm/lib/ExecutionEngine/Orc/LLJIT.cpp
index 0a0faaa..fef190b 100644
--- a/llvm/lib/ExecutionEngine/Orc/LLJIT.cpp
+++ b/llvm/lib/ExecutionEngine/Orc/LLJIT.cpp
@@ -83,7 +83,7 @@
 
 LLJIT::LLJIT(std::unique_ptr<ExecutionSession> ES,
              std::unique_ptr<TargetMachine> TM, DataLayout DL)
-    : ES(std::move(ES)), Main(this->ES->createJITDylib("main")),
+    : ES(std::move(ES)), Main(this->ES->getMainJITDylib()),
       DL(std::move(DL)),
       ObjLinkingLayer(*this->ES,
                       [this](VModuleKey K) { return getMemoryManager(K); }),
@@ -93,7 +93,7 @@
 LLJIT::LLJIT(std::unique_ptr<ExecutionSession> ES,
              JITTargetMachineBuilder JTMB, DataLayout DL,
              unsigned NumCompileThreads)
-    : ES(std::move(ES)), Main(this->ES->createJITDylib("main")),
+    : ES(std::move(ES)), Main(this->ES->getMainJITDylib()),
       DL(std::move(DL)),
       ObjLinkingLayer(*this->ES,
                       [this](VModuleKey K) { return getMemoryManager(K); }),