Split Finish into Finish and FinishImpl to have a common place to do end of

file error checking. Use that to error on an unfinished cfi_startproc.

The error is not nice, but is already better than a segmentation fault.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147717 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCStreamer.h

@@ -590,8 +590,10 @@ namespace llvm {
     virtual void EmitRegSave(const SmallVectorImpl<unsigned> &RegList,
                              bool isVector);
 
+    /// FinishImpl - Streamer specific finalization.
+    virtual void FinishImpl() = 0;
     /// Finish - Finish emission of machine code.
-    virtual void Finish() = 0;
+    void Finish();
   };
 
   /// createNullStreamer - Create a dummy machine code streamer, which does