TableGen: Use StringRef instead of const std::string& in return vals.

This will allow to switch to a different string storage in an upcoming
commit.

llvm-svn: 288612
diff --git a/llvm/utils/TableGen/InstrInfoEmitter.cpp b/llvm/utils/TableGen/InstrInfoEmitter.cpp
index e9c448d..a7abf61 100644
--- a/llvm/utils/TableGen/InstrInfoEmitter.cpp
+++ b/llvm/utils/TableGen/InstrInfoEmitter.cpp
@@ -217,7 +217,8 @@
       }
       OpList[I->second] = Info.MIOperandNo;
     }
-    OperandMap[OpList].push_back(Namespace + "::" + Inst->TheDef->getName());
+    OperandMap[OpList].push_back(Namespace + "::" +
+                                 Inst->TheDef->getName().str());
   }
 }