[TableGen] Use StringRef instead of std::string for CodeGenInstruction namespace. NFC

llvm-svn: 307362
diff --git a/llvm/utils/TableGen/CodeGenTarget.cpp b/llvm/utils/TableGen/CodeGenTarget.cpp
index eb277f3..58df3ce 100644
--- a/llvm/utils/TableGen/CodeGenTarget.cpp
+++ b/llvm/utils/TableGen/CodeGenTarget.cpp
@@ -207,7 +207,7 @@
   return TargetRec->getName();
 }
 
-std::string CodeGenTarget::getInstNamespace() const {
+StringRef CodeGenTarget::getInstNamespace() const {
   for (const CodeGenInstruction *Inst : getInstructionsByEnumValue()) {
     // Make sure not to pick up "TargetOpcode" by accidentally getting
     // the namespace off the PHI instruction or something.