[llvm-mc-assemble-fuzzer] Update API - Pass MCObjectWriter instead of a stream

[pbhatu]

Summary

Fixes build breakage of llvm-mc-assemble-fuzzer introduced by r332749.

[bcain]

LGTM.

Should we have this build configuration in a buildbot?

[pbhatu]

I guess a buildbot should exist for this as this build failure was undetected for almost 3 months.
Anyways, since I don't have commit access, could you please commit it for me?

[bcain]

r339981

tools/llvm-mc-assemble-fuzzer/llvm-mc-assemble-fuzzer.cpp

 #include "llvm/MC/SubtargetFeature.h"
 #include "llvm/MC/MCAsmBackend.h"
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/MC/MCCodeEmitter.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCInstPrinter.h"
 #include "llvm/MC/MCInstrInfo.h"
 #include "llvm/MC/MCObjectFileInfo.h"
+#include "llvm/MC/MCObjectWriter.h"
 #include "llvm/MC/MCParser/AsmLexer.h"
 #include "llvm/MC/MCParser/MCTargetAsmParser.h"
 #include "llvm/MC/MCRegisterInfo.h"
 #include "llvm/MC/MCSectionMachO.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCSubtargetInfo.h"
 #include "llvm/MC/MCTargetOptionsCommandFlags.inc"
 #include "llvm/Support/MemoryBuffer.h"
if (FileType == OFT_AssemblyFile) {
     if (!Out->os().supportsSeeking()) {
       BOS = make_unique<buffer_ostream>(Out->os());
       OS = BOS.get();
     }
 
     MCCodeEmitter *CE = TheTarget->createMCCodeEmitter(*MCII, *MRI, Ctx);
     MCAsmBackend *MAB = TheTarget->createMCAsmBackend(*STI, *MRI, MCOptions);
     Str.reset(TheTarget->createMCObjectStreamer(
-        TheTriple, Ctx, std::unique_ptr<MCAsmBackend>(MAB), *OS,
-        std::unique_ptr<MCCodeEmitter>(CE), *STI, MCOptions.MCRelaxAll,
-        MCOptions.MCIncrementalLinkerCompatible,
+        TheTriple, Ctx, std::unique_ptr<MCAsmBackend>(MAB),
+        MAB->createObjectWriter(*OS), std::unique_ptr<MCCodeEmitter>(CE), *STI,
+        MCOptions.MCRelaxAll, MCOptions.MCIncrementalLinkerCompatible,
         /*DWARFMustBeAtTheEnd*/ false));
   }
   const int Res = AssembleInput(ProgName, TheTarget, SrcMgr, Ctx, *Str, *MAI, *STI,
       *MCII, MCOptions);
 
   (void) Res;
 
   return 0;