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/FixedLenDecoderEmitter.cpp b/llvm/utils/TableGen/FixedLenDecoderEmitter.cpp
index a20b469..010f415 100644
--- a/llvm/utils/TableGen/FixedLenDecoderEmitter.cpp
+++ b/llvm/utils/TableGen/FixedLenDecoderEmitter.cpp
@@ -400,7 +400,7 @@
   }
 
   // Returns the record name.
-  const std::string &nameWithID(unsigned Opcode) const {
+  const StringRef nameWithID(unsigned Opcode) const {
     return AllInstructions[Opcode]->TheDef->getName();
   }
 
@@ -1720,7 +1720,7 @@
     TypeRecord = TypeRecord->getValueAsDef("RegClass");
 
   if (TypeRecord->isSubClassOf("RegisterClass")) {
-    Decoder = "Decode" + TypeRecord->getName() + "RegisterClass";
+    Decoder = "Decode" + TypeRecord->getName().str() + "RegisterClass";
   } else if (TypeRecord->isSubClassOf("PointerLikeRegClass")) {
     Decoder = "DecodePointerLikeRegClass" +
       utostr(TypeRecord->getValueAsInt("RegClassKind"));
@@ -1889,7 +1889,7 @@
       if (TypeRecord->isSubClassOf("RegisterOperand"))
         TypeRecord = TypeRecord->getValueAsDef("RegClass");
       if (TypeRecord->isSubClassOf("RegisterClass")) {
-        Decoder = "Decode" + TypeRecord->getName() + "RegisterClass";
+        Decoder = "Decode" + TypeRecord->getName().str() + "RegisterClass";
         isReg = true;
       } else if (TypeRecord->isSubClassOf("PointerLikeRegClass")) {
         Decoder = "DecodePointerLikeRegClass" +