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

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.

Differential Revision: https://reviews.llvm.org/D47050

llvm-svn: 332749
diff --git a/llvm/lib/MC/MCWasmStreamer.cpp b/llvm/lib/MC/MCWasmStreamer.cpp
index 984f491..a74d375 100644
--- a/llvm/lib/MC/MCWasmStreamer.cpp
+++ b/llvm/lib/MC/MCWasmStreamer.cpp
@@ -198,11 +198,11 @@
 
 MCStreamer *llvm::createWasmStreamer(MCContext &Context,
                                      std::unique_ptr<MCAsmBackend> &&MAB,
-                                     raw_pwrite_stream &OS,
+                                     std::unique_ptr<MCObjectWriter> &&OW,
                                      std::unique_ptr<MCCodeEmitter> &&CE,
                                      bool RelaxAll) {
   MCWasmStreamer *S =
-      new MCWasmStreamer(Context, std::move(MAB), OS, std::move(CE));
+      new MCWasmStreamer(Context, std::move(MAB), std::move(OW), std::move(CE));
   if (RelaxAll)
     S->getAssembler().setRelaxAll(true);
   return S;