[MC] Add emitXCOFFSymbolLinkageWithVisibility to MCNullStreamer

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

Reviewed By: DiggerLin

Differential Revision: https://reviews.llvm.org/D114876

Author: jakeegan

llvm/lib/MC/MCNullStreamer.cpp

@@ -40,6 +40,9 @@ namespace {
     void EmitCOFFSymbolStorageClass(int StorageClass) override {}
     void EmitCOFFSymbolType(int Type) override {}
     void EndCOFFSymbolDef() override {}
+    void
+    emitXCOFFSymbolLinkageWithVisibility(MCSymbol *Symbol, MCSymbolAttr Linkage,
+                                         MCSymbolAttr Visibility) override {}
   };
 
 }