Add capability to push/pop DFI in MCStreamer. NFC

[deadalnix]

Summary

This is extracted from D17555

[davidxl]

Since the push/pop methods are not used so it does not really belong to the refactoring (can be left in the original patch). Otherwise ltgm.

[deadalnix]

Remove push/pop

include/llvm/MC/MCStreamer.h

MCTargetStreamer *getTargetStreamer() {
     return TargetStreamer.get();
   }
 
   unsigned getNumFrameInfos() { return DwarfFrameInfos.size(); }
   ArrayRef<MCDwarfFrameInfo> getDwarfFrameInfos() const {
     return DwarfFrameInfos;
   }
 
+  bool hasUnfinishedDwarfFrameInfo();
+
   unsigned getNumWinFrameInfos() { return WinFrameInfos.size(); }
   ArrayRef<WinEH::FrameInfo *> getWinFrameInfos() const {
     return WinFrameInfos;
   }
 
   void generateCompactUnwindEncodings(MCAsmBackend *MAB);
 
   /// \name Assembly File Formatting.

lib/MC/MCStreamer.cpp

 }
 
 MCDwarfFrameInfo *MCStreamer::getCurrentDwarfFrameInfo() {
   if (DwarfFrameInfos.empty())
     return nullptr;
   return &DwarfFrameInfos.back();
 }
 
+bool MCStreamer::hasUnfinishedDwarfFrameInfo() {
+  MCDwarfFrameInfo *CurFrame = getCurrentDwarfFrameInfo();
+  return CurFrame && !CurFrame->End;
+}
+
 void MCStreamer::EnsureValidDwarfFrame() {
   MCDwarfFrameInfo *CurFrame = getCurrentDwarfFrameInfo();
   if (!CurFrame || CurFrame->End)
     report_fatal_error("No open frame");
 }
 
 unsigned MCStreamer::EmitCVFileDirective(unsigned FileNo, StringRef Filename) {
   return getContext().getCVFile(Filename, FileNo);
if (TS)
     TS->emitLabel(Symbol);
 }
 
 void MCStreamer::EmitCFISections(bool EH, bool Debug) {
   assert(EH || Debug);
 }
 
 void MCStreamer::EmitCFIStartProc(bool IsSimple) {
-  MCDwarfFrameInfo *CurFrame = getCurrentDwarfFrameInfo();
-  if (CurFrame && !CurFrame->End)
+  if (hasUnfinishedDwarfFrameInfo())
     report_fatal_error("Starting a frame before finishing the previous one!");
 
   MCDwarfFrameInfo Frame;
   Frame.IsSimple = IsSimple;
   EmitCFIStartProcImpl(Frame);
 
   const MCAsmInfo* MAI = Context.getAsmInfo();
   if (MAI) {