[MC] Plumb unique_ptr<MCELFObjectTargetWriter> through createELFObjectWriter to
ELFObjectWriter's constructor.

Fixes the same ownership issue for ELF that r315245 did for MachO:
ELFObjectWriter takes ownership of its MCELFObjectTargetWriter, so we want to
pass this through to the constructor via a unique_ptr, rather than a raw ptr.

llvm-svn: 315254
diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
index dc89347..6f825a3 100644
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
@@ -238,6 +238,6 @@
 MCObjectWriter *llvm::createARMELFObjectWriter(raw_pwrite_stream &OS,
                                                uint8_t OSABI,
                                                bool IsLittleEndian) {
-  MCELFObjectTargetWriter *MOTW = new ARMELFObjectWriter(OSABI);
-  return createELFObjectWriter(MOTW, OS, IsLittleEndian);
+  return createELFObjectWriter(llvm::make_unique<ARMELFObjectWriter>(OSABI), OS,
+                               IsLittleEndian);
 }