[ORC] Create a new SymbolStringPool by default in ExecutionSession constructor.

This makes the common case of constructing an ExecutionSession tidier.

llvm-svn: 329013
diff --git a/llvm/unittests/ExecutionEngine/Orc/CompileOnDemandLayerTest.cpp b/llvm/unittests/ExecutionEngine/Orc/CompileOnDemandLayerTest.cpp
index 4501b53..833b762 100644
--- a/llvm/unittests/ExecutionEngine/Orc/CompileOnDemandLayerTest.cpp
+++ b/llvm/unittests/ExecutionEngine/Orc/CompileOnDemandLayerTest.cpp
@@ -59,8 +59,7 @@
 
   DummyCallbackManager CallbackMgr;
 
-  SymbolStringPool SSP;
-  ExecutionSession ES(SSP);
+  ExecutionSession ES(std::make_shared<SymbolStringPool>());
 
   auto GetResolver =
       [](orc::VModuleKey) -> std::shared_ptr<llvm::orc::SymbolResolver> {