[MC] Thread unique_ptr<MCObjectWriter> through the create.*ObjectWriter
functions.

This makes the ownership of the resulting MCObjectWriter clear, and allows us
to remove one instance of MCObjectStreamer's bizarre "holding ownership via
someone else's reference" trick.

llvm-svn: 315327
diff --git a/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCObjectWriter.cpp b/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCObjectWriter.cpp
index 166a3f0..238926d 100644
--- a/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCObjectWriter.cpp
+++ b/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCObjectWriter.cpp
@@ -13,6 +13,7 @@
 #include "llvm/MC/MCELFObjectWriter.h"
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCFixup.h"
+#include "llvm/MC/MCObjectWriter.h"
 #include "llvm/MC/MCValue.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <cassert>
@@ -160,8 +161,8 @@
   }
 }
 
-MCObjectWriter *llvm::createSystemZObjectWriter(raw_pwrite_stream &OS,
-                                                uint8_t OSABI) {
+std::unique_ptr<MCObjectWriter>
+llvm::createSystemZObjectWriter(raw_pwrite_stream &OS, uint8_t OSABI) {
   return createELFObjectWriter(llvm::make_unique<SystemZObjectWriter>(OSABI),
                                OS, /*IsLittleEndian=*/false);
 }