ArrayRef'ize restoreCalleeSavedRegisters. NFCI.

restoreCalleeSavedRegisters can mutate the contents of the
CalleeSavedInfos, so use a MutableArrayRef.
diff --git a/llvm/lib/Target/ARC/ARCFrameLowering.h b/llvm/lib/Target/ARC/ARCFrameLowering.h
index a96bf8e..9951a09 100644
--- a/llvm/lib/Target/ARC/ARCFrameLowering.h
+++ b/llvm/lib/Target/ARC/ARCFrameLowering.h
@@ -48,7 +48,7 @@
   bool
   restoreCalleeSavedRegisters(MachineBasicBlock &MBB,
                               MachineBasicBlock::iterator MI,
-                              std::vector<CalleeSavedInfo> &CSI,
+                              MutableArrayRef<CalleeSavedInfo> CSI,
                               const TargetRegisterInfo *TRI) const override;
 
   void processFunctionBeforeFrameFinalized(MachineFunction &MF,