[MC] Plumb unique_ptr<MCWasmObjectTargetWriter> through createWasmObjectWriter
to WasmObjectWriter's constructor.

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

llvm-svn: 315260
diff --git a/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyWasmObjectWriter.cpp b/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyWasmObjectWriter.cpp
index 995984b..af97319 100644
--- a/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyWasmObjectWriter.cpp
+++ b/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyWasmObjectWriter.cpp
@@ -95,6 +95,6 @@
 
 MCObjectWriter *llvm::createWebAssemblyWasmObjectWriter(raw_pwrite_stream &OS,
                                                         bool Is64Bit) {
-  MCWasmObjectTargetWriter *MOTW = new WebAssemblyWasmObjectWriter(Is64Bit);
-  return createWasmObjectWriter(MOTW, OS);
+  auto MOTW = llvm::make_unique<WebAssemblyWasmObjectWriter>(Is64Bit);
+  return createWasmObjectWriter(std::move(MOTW), OS);
 }