Allow a target to create a null streamer.

Targets can assume that a target streamer is present, so they have to be able
to construct a null streamer in order to set the target streamer in it to.

Fixes a crash when using the null streamer with arm.

llvm-svn: 211358
diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp
index a4d13ed..66a2d59 100644
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp
@@ -1332,6 +1332,12 @@
   return S;
 }
 
+MCStreamer *createARMNullStreamer(MCContext &Ctx) {
+  MCStreamer *S = llvm::createNullStreamer(Ctx);
+  new ARMTargetStreamer(*S);
+  return S;
+}
+
   MCELFStreamer* createARMELFStreamer(MCContext &Context, MCAsmBackend &TAB,
                                       raw_ostream &OS, MCCodeEmitter *Emitter,
                                       bool RelaxAll, bool NoExecStack,