llvm-ml: Rework the alias directive to not use ELF-intended VK_WEAKREF

Set `WeakExternal` to disable the the expansion in MCExpr.cpp:canExpand.

Author: MaskRay

llvm/lib/MC/MCExpr.cpp

@@ -478,13 +478,7 @@ static bool canExpand(const MCSymbol &Sym, bool InSet) {
   if (Sym.isWeakExternal())
     return false;
 
-  const MCExpr *Expr = Sym.getVariableValue(true);
-  // Special case for llvm-ml alias
-  const auto *Inner = dyn_cast<MCSymbolRefExpr>(Expr);
-  if (Inner) {
-    if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF)
-      return false;
-  }
+  Sym.getVariableValue(true);
 
   if (InSet)
     return true;

llvm/lib/MC/MCWinCOFFStreamer.cpp

@@ -414,14 +414,15 @@ void MCWinCOFFStreamer::emitLocalCommonSymbol(MCSymbol *S, uint64_t Size,
   popSection();
 }
 
+// Hack: Used by llvm-ml to implement the alias directive.
 void MCWinCOFFStreamer::emitWeakReference(MCSymbol *AliasS,
                                           const MCSymbol *Symbol) {
   auto *Alias = cast<MCSymbolCOFF>(AliasS);
   emitSymbolAttribute(Alias, MCSA_Weak);
+  Alias->setIsWeakExternal(true);
 
   getAssembler().registerSymbol(*Symbol);
-  Alias->setVariableValue(MCSymbolRefExpr::create(
-      Symbol, MCSymbolRefExpr::VK_WEAKREF, getContext()));
+  Alias->setVariableValue(MCSymbolRefExpr::create(Symbol, getContext()));
 }
 
 // TODO: Implement this if you want to emit .comment section in COFF obj files.