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

llvm-svn: 307362
diff --git a/llvm/utils/TableGen/CodeGenInstruction.h b/llvm/utils/TableGen/CodeGenInstruction.h
index 75db17b..e173e15 100644
--- a/llvm/utils/TableGen/CodeGenInstruction.h
+++ b/llvm/utils/TableGen/CodeGenInstruction.h
@@ -206,7 +206,7 @@
   class CodeGenInstruction {
   public:
     Record *TheDef;            // The actual record defining this instruction.
-    std::string Namespace;     // The namespace the instruction is in.
+    StringRef Namespace;       // The namespace the instruction is in.
 
     /// AsmString - The format string used to emit a .s file for the
     /// instruction.