[WebAssembly] Allow non-zero table offset in input object

[sbc100]

Summary

This change enables D42284 to land without breaking lld

[ncw]

We're creating work for ourselves a bit here... if #2 were landed before #3, then this change wouldn't be needed. The "significant" patches are #2 and #5, and the others are mainly just for neatness - by doing 3/4/6 first it needs more of these little bits of sticking plaster.

It's OK though, this patch doesn't do any harm if you prefer this order.

lld/trunk/wasm/InputFiles.cpp

void ObjFile::initializeSymbols() {
   if (SegmentCount) {
     if (SegmentCount > 1)
       fatal(getName() + ": contains more than one element segment");
     const WasmElemSegment &Segment = WasmObj->elements()[0];
     if (Segment.Offset.Opcode != WASM_OPCODE_I32_CONST)
       fatal(getName() + ": unsupported element segment");
     if (Segment.TableIndex != 0)
       fatal(getName() + ": unsupported table index in elem segment");
-    if (Segment.Offset.Value.Int32 != 0)
-      fatal(getName() + ": unsupported element segment offset");
-    TableSymbols.reserve(Segment.Functions.size());
+    uint32_t Offset = Segment.Offset.Value.Int32;
+    TableSymbols.resize(Offset);
+    TableSymbols.reserve(Offset + Segment.Functions.size());
     for (uint64_t FunctionIndex : Segment.Functions)
       TableSymbols.push_back(FunctionSymbols[FunctionIndex]);
   }
 
   DEBUG(dbgs() << "TableSymbols: " << TableSymbols.size() << "\n");
   DEBUG(dbgs() << "Functions   : " << FunctionSymbols.size() << "\n");
   DEBUG(dbgs() << "Globals     : " << GlobalSymbols.size() << "\n");
 }

lld/trunk/wasm/Writer.cpp

for (InputFunction *Func : File->Functions) {
       DefinedFunctions.emplace_back(Func);
       Func->setOutputIndex(FunctionIndex++);
     }
   }
 
   for (ObjFile *File : Symtab->ObjectFiles) {
     DEBUG(dbgs() << "Table Indexes: " << File->getName() << "\n");
     for (Symbol *Sym : File->getTableSymbols()) {
-      if (Sym->hasTableIndex() || !Sym->hasOutputIndex())
+      if (!Sym || Sym->hasTableIndex() || !Sym->hasOutputIndex())
         continue;
       Sym->setTableIndex(TableIndex++);
       IndirectFunctions.emplace_back(Sym);
     }
   }
 }
 
 static StringRef getOutputDataSegmentName(StringRef Name) {