[ORC] Pass symbol name to discard by const reference.

This saves some unnecessary atomic ref-counting operations.

llvm-svn: 343927
diff --git a/llvm/lib/ExecutionEngine/Orc/CompileOnDemandLayer.cpp b/llvm/lib/ExecutionEngine/Orc/CompileOnDemandLayer.cpp
index 78ee0e7..368969b 100644
--- a/llvm/lib/ExecutionEngine/Orc/CompileOnDemandLayer.cpp
+++ b/llvm/lib/ExecutionEngine/Orc/CompileOnDemandLayer.cpp
@@ -85,7 +85,7 @@
                          std::move(SymbolToDefinition));
   }
 
-  void discard(const JITDylib &V, SymbolStringPtr Name) override {
+  void discard(const JITDylib &V, const SymbolStringPtr &Name) override {
     // All original symbols were materialized by the CODLayer and should be
     // final. The function bodies provided by M should never be overridden.
     llvm_unreachable("Discard should never be called on an "