[MC] Add emitXCOFFSymbolLinkageWithVisibility to MCNullStreamer

[Jake-Egan]

Summary

This patch adds emitXCOFFSymbolLinkageWithVisibility to MCNullStreamer to fix llvm_unreachable getting reached when using option -filetype=null on AIX.

[DiggerLin]

LGTM

llvm/lib/MC/MCNullStreamer.cpp

public:
     void emitZerofill(MCSection *Section, MCSymbol *Symbol = nullptr,
                       uint64_t Size = 0, unsigned ByteAlignment = 0,
                       SMLoc Loc = SMLoc()) override {}
     void emitGPRel32Value(const MCExpr *Value) override {}
     void BeginCOFFSymbolDef(const MCSymbol *Symbol) override {}
     void EmitCOFFSymbolStorageClass(int StorageClass) override {}
     void EmitCOFFSymbolType(int Type) override {}
     void EndCOFFSymbolDef() override {}
+    void
+    emitXCOFFSymbolLinkageWithVisibility(MCSymbol *Symbol, MCSymbolAttr Linkage,
+                                         MCSymbolAttr Visibility) override {}
   };
 
 }
 
 MCStreamer *llvm::createNullStreamer(MCContext &Context) {
   return new MCNullStreamer(Context);
 }