[WebAssembly] Add check to ensure symbol VA is only set once. NFC

[sbc100]

Summary

Also remove resulting unneeded function.

[ruiu]

LGTM

wasm/Driver.cpp

 static Optional<std::string> findFile(StringRef Path1, const Twine &Path2) {
   SmallString<128> S;
   path::append(S, Path1, Path2);
   if (fs::exists(S))
     return S.str().str();
   return None;
 }
 
-// Inject a new wasm global into the output binary with the given value.
-// Wasm global are used in relocatable object files to model symbol imports
-// and exports.  In the final executable the only use of wasm globals is
-// for the exlicit stack pointer (__stack_pointer).
-static Symbol* addSyntheticGlobal(StringRef Name, int32_t Value) {
-  log("injecting global: " + Name);
-  Symbol *S = Symtab->addDefinedGlobal(Name);
-  S->setVirtualAddress(Value);
-  return S;
-}
-
 // Inject a new undefined symbol into the link.  This will cause the link to
 // fail unless this symbol can be found.
 static void addSyntheticUndefinedFunction(StringRef Name,
                                           const WasmSignature *Type) {
   log("injecting undefined func: " + Name);
   Symtab->addUndefinedFunction(Name, Type);
 }
 
if (Config->Entry.empty())
       Config->Entry = "_start";
     static WasmSignature Signature = {{}, WASM_TYPE_NORESULT};
     addSyntheticUndefinedFunction(Config->Entry, &Signature);
 
     // Handle the `--undefined <sym>` options.
     for (StringRef S : args::getStrings(Args, OPT_undefined))
       addSyntheticUndefinedFunction(S, nullptr);
 
-    Config->StackPointerSymbol = addSyntheticGlobal("__stack_pointer", 0);
+    Config->StackPointerSymbol = Symtab->addDefinedGlobal("__stack_pointer");
   }
 
   createFiles(Args);
   if (errorCount())
     return;
 
   // Add all files to the symbol table. This will add almost all
   // symbols that we need to the symbol table.

wasm/Symbols.cpp

 uint32_t Symbol::getOutputIndex() const {
   if (isUndefined() && isWeak())
     return 0;
   return OutputIndex.getValue();
 }
 
 void Symbol::setVirtualAddress(uint32_t Value) {
   DEBUG(dbgs() << "setVirtualAddress " << Name << " -> " << Value << "\n");
+  assert(!VirtualAddress.hasValue());
   VirtualAddress = Value;
 }
 
 void Symbol::setOutputIndex(uint32_t Index) {
   DEBUG(dbgs() << "setOutputIndex " << Name << " -> " << Index << "\n");
   assert(!hasOutputIndex());
   OutputIndex = Index;
 }