[RISCV][NFC] Move some std::string to StringRef

llvm-svn: 354333
diff --git a/llvm/lib/Target/RISCV/RISCVSubtarget.h b/llvm/lib/Target/RISCV/RISCVSubtarget.h
index 66c7730..0373d54 100644
--- a/llvm/lib/Target/RISCV/RISCVSubtarget.h
+++ b/llvm/lib/Target/RISCV/RISCVSubtarget.h
@@ -51,8 +51,8 @@
 
 public:
   // Initializes the data members to match that of the specified triple.
-  RISCVSubtarget(const Triple &TT, const std::string &CPU,
-                 const std::string &FS, const TargetMachine &TM);
+  RISCVSubtarget(const Triple &TT, StringRef CPU, StringRef FS,
+                 const TargetMachine &TM);
 
   // Parses features string setting specified subtarget options. The
   // definition of this function is auto-generated by tblgen.