[WebAssembly] Base imports on Symtab. NFC.

[sbc100]

Summary

Since imports are undefined symbols we know we can
find all of them my looking at the symbol table alone.
(i.e. imports cannot be have local binding).

This will be strictly faster and also allows us
to to remove a method from Symbol class

[ruiu]

LGTM

lld/trunk/wasm/Symbols.h

public:
   const WasmSignature &getFunctionType() const;
   uint32_t getOutputIndex() const;
   uint32_t getTableIndex() const { return TableIndex.getValue(); }
 
   // Returns the virtual address of a defined global.
   // Only works for globals, not functions.
   uint32_t getVirtualAddress() const;
 
-  // Returns true if an output index has been set for this symbol
-  bool hasOutputIndex() const { return OutputIndex.hasValue(); }
-
   // Set the output index of the symbol (in the function or global index
   // space of the output object.
   void setOutputIndex(uint32_t Index);
 
   // Returns true if a table index has been set for this symbol
   bool hasTableIndex() const { return TableIndex.hasValue(); }
 
   // Set the table index of the symbol

lld/trunk/wasm/Writer.cpp

for (ObjFile *File : Symtab->ObjectFiles) {
 
     // Memory
     if (WasmFile->memories().size() > 1)
       fatal(File->getName() + ": contains more than one memory");
   }
 }
 
 void Writer::calculateImports() {
-  for (ObjFile *File : Symtab->ObjectFiles) {
-    for (Symbol *Sym : File->getSymbols()) {
-      if (Sym->hasOutputIndex() || Sym->isDefined() || Sym->isWeak())
-        continue;
+  for (Symbol *Sym : Symtab->getSymbols()) {
+    if (Sym->isDefined() || Sym->isWeak())
 
-      if (Sym->isFunction()) {
-        Sym->setOutputIndex(FunctionImports.size());
-        FunctionImports.push_back(Sym);
-      } else {
-        Sym->setOutputIndex(GlobalImports.size());
-        GlobalImports.push_back(Sym);
-      }
-    }
-  }
-}
 
 uint32_t Writer::getTypeIndex(const WasmSignature &Sig) {
   auto Pair = TypeIndices.insert(std::make_pair(Sig, Types.size()));
   if (Pair.second)
     Types.push_back(&Sig);
   return Pair.first->second;
 }