MC: Change the streamer ctors to take an object writer instead of a stream. NFCI.

[pcc]

Summary

The idea is that a client that wants split dwarf would create a
specific kind of object writer that creates two files, and use it to
create the streamer.

Part of PR37466.

[JDevlieghere]

LGMT

[echristo]

LGTM.

tools/driver/cc1as_main.cpp

 #include "llvm/ADT/Triple.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/MC/MCAsmBackend.h"
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/MC/MCCodeEmitter.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCInstrInfo.h"
 #include "llvm/MC/MCObjectFileInfo.h"
+#include "llvm/MC/MCObjectWriter.h"
 #include "llvm/MC/MCParser/MCAsmParser.h"
 #include "llvm/MC/MCParser/MCTargetAsmParser.h"
 #include "llvm/MC/MCRegisterInfo.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCSubtargetInfo.h"
 #include "llvm/MC/MCTargetOptions.h"
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
if (!FDOS->supportsSeeking()) {
       Out = BOS.get();
     }
 
     std::unique_ptr<MCCodeEmitter> CE(
         TheTarget->createMCCodeEmitter(*MCII, *MRI, Ctx));
     MCTargetOptions MCOptions;
     std::unique_ptr<MCAsmBackend> MAB(
         TheTarget->createMCAsmBackend(*STI, *MRI, MCOptions));
+    std::unique_ptr<MCObjectWriter> OW = MAB->createObjectWriter(*Out);
 
     Triple T(Opts.Triple);
     Str.reset(TheTarget->createMCObjectStreamer(
-        T, Ctx, std::move(MAB), *Out, std::move(CE), *STI, Opts.RelaxAll,
-        Opts.IncrementalLinkerCompatible,
+        T, Ctx, std::move(MAB), std::move(OW), std::move(CE), *STI,
+        Opts.RelaxAll, Opts.IncrementalLinkerCompatible,
         /*DWARFMustBeAtTheEnd*/ true));
     Str.get()->InitSections(Opts.NoExecStack);
   }
 
   // Assembly to object compilation should leverage assembly info.
   Str->setUseAssemblerInfoForParsing(true);
 
   bool Failed = false;