[WebAssembly] Add LLD support for defined globals

[ncw]

Summary

Makes use of the new constants added in D42752. Comes after D42585.

Probably should be updated with an extra test - but I'm pretty sure it is working.

[ncw]

Abandoning; COMDAT for globals would be nice (for completeness) but isn't important now - no need to fight for it currently.

I still think it should probably be added to Linking.md before the first formally-complete release, but I agree it's not important.

wasm/InputChunks.h

InputGlobal(const WasmGlobalType &Signature, const WasmGlobal *G,
               const ObjFile *F)
       : InputChunk(F, InputChunk::Global), Signature(Signature), Global(G) {}
 
   static bool classof(const InputChunk *C) {
     return C->kind() == InputChunk::Global;
   }
 
   virtual const WasmInitExpr &initExpr() const { return Global->InitExpr; }
-  StringRef getComdat() const override { return StringRef(); }
+  StringRef getComdat() const override { return Global->Comdat; }
   uint32_t getOutputIndex() const override { return OutputIndex.getValue(); }
   bool hasOutputIndex() const override { return OutputIndex.hasValue(); }
   void setOutputIndex(uint32_t Index);
 
   const WasmGlobalType &Signature;
 
 protected:
   StringRef getName() const override { return {}; }

wasm/InputFiles.cpp

case WasmSymbol::SymbolType::DATA:
         }
       }
       S = createUndefined(WasmSym, Symbol::Kind::UndefinedDataKind);
       break;
 
     case WasmSymbol::SymbolType::GLOBAL:
       if (WasmSym.WasmIndex >= NumGlobalImports) {
         InputGlobal *Global = getGlobal(WasmSym);
+        if (!isExcludedByComdat(Global)) {
           S = createDefined(WasmSym, Symbol::Kind::DefinedGlobalKind, Global);
           break;
+        } else {
+          Global->Discarded = true;
+          // Exclude global, and add the symbol as undefined
+        }
       }
       S = createUndefined(WasmSym, Symbol::Kind::UndefinedGlobalKind);
       break;
     }
 
     Symbols.push_back(S);
   }
 }

wasm/Writer.cpp

void Writer::createLinkingSection() {
   std::map<StringRef,std::vector<ComdatEntry>> Comdats;
 
   for (const InputFunction *F : DefinedFunctions) {
     StringRef Comdat = F->getComdat();
     if (!Comdat.empty())
       Comdats[Comdat].emplace_back(
           ComdatEntry{WASM_COMDAT_FUNCTION, F->getOutputIndex()});
   }
+  for (const InputGlobal *G : DefinedGlobals) {
+    StringRef Comdat = G->getComdat();
+    if (!Comdat.empty())
+      Comdats[Comdat].emplace_back(
+          ComdatEntry{WASM_COMDAT_GLOBAL, G->getOutputIndex()});
+  }
   for (uint32_t I = 0; I < Segments.size(); ++I) {
     const auto &InputSegments = Segments[I]->InputSegments;
     if (InputSegments.empty())
       continue;
     StringRef Comdat = InputSegments[0]->getComdat();
 #ifndef NDEBUG
     for (const InputSegment *IS : InputSegments)
       assert(IS->getComdat() == Comdat);