[WebAssmebly] Fix references to weak aliases

[sbc100]

Summary

Corresponding LLVM change: https://reviews.llvm.org/D41472

lld/trunk/test/wasm/weak-alias-overide.ll

 ; CHECK-NEXT:       - Name:            call_direct_ptr
 ; CHECK-NEXT:         Kind:            FUNCTION
 ; CHECK-NEXT:         Index:           6
 ; CHECK-NEXT:   - Type:            ELEM
 ; CHECK-NEXT:     Segments:        
 ; CHECK-NEXT:       - Offset:          
 ; CHECK-NEXT:           Opcode:          I32_CONST
 ; CHECK-NEXT:           Value:           1
-; CHECK-NEXT:         Functions:       [ 0 ]
+; CHECK-NEXT:         Functions:       [ 2 ]
 ; CHECK-NEXT:   - Type:            CODE
 ; CHECK-NEXT:     Functions:
 ; CHECK-NEXT:       - Locals:
 ; CHECK-NEXT:         Body:            41010B
 ; CHECK-NEXT:       - Locals:
 ; CHECK-NEXT:         Body:            1080808080001A0B
 ; CHECK-NEXT:       - Locals:
 ; CHECK-NEXT:         Body:            41000B
 ; CHECK-NEXT:       - Locals:
-; CHECK-NEXT:         Body:            1080808080000B
+; CHECK-NEXT:         Body:            1082808080000B
 ; CHECK-NEXT:       - Locals:
 ; CHECK-NEXT:         Body:            1080808080000B
 ; CHECK-NEXT:       - Locals:          
 ; CHECK-NEXT:         Body:            41010B
 ; CHECK-NEXT:       - Locals:          
 ; CHECK-NEXT:           - Type:            I32
 ; CHECK-NEXT:             Count:           2
-; CHECK-NEXT:         Body:            23808080800041106B220024808080800020004181808080003602081080808080002101200041106A24808080800020010B
+; CHECK-NEXT:         Body:            23808080800041106B220024808080800020004181808080003602081082808080002101200041106A24808080800020010B
 ; CHECK-NEXT:   - Type:            CUSTOM
 ; CHECK-NEXT:     Name:            linking
 ; CHECK-NEXT:     DataSize:        0
 ; CHECK-NEXT:   - Type:            CUSTOM
 ; CHECK-NEXT:     Name:            name
 ; CHECK-NEXT:     FunctionNames:
 ; CHECK-NEXT:       - Index:           0
 ; CHECK-NEXT:         Name:            alias_fn

lld/trunk/wasm/InputFiles.cpp

case WasmSymbol::SymbolType::DEBUG_FUNCTION_NAME:
       continue;
     }
 
     Symbols.push_back(S);
     if (WasmSym.isFunction()) {
       DEBUG(dbgs() << "Function: " << WasmSym.ElementIndex << " -> "
                    << toString(*S) << "\n");
       FunctionSymbols[WasmSym.ElementIndex] = S;
+      if (WasmSym.HasAltIndex)
+        FunctionSymbols[WasmSym.AltIndex] = S;
     } else {
       DEBUG(dbgs() << "Global: " << WasmSym.ElementIndex << " -> "
                    << toString(*S) << "\n");
       GlobalSymbols[WasmSym.ElementIndex] = S;
+      if (WasmSym.HasAltIndex)
+        GlobalSymbols[WasmSym.AltIndex] = S;
     }
   }
 
+  DEBUG(for (size_t I = 0; I < FunctionSymbols.size(); ++I)
+            assert(FunctionSymbols[I] != nullptr);
+        for (size_t I = 0; I < GlobalSymbols.size(); ++I)
+            assert(GlobalSymbols[I] != nullptr););
+
   // Populate `TableSymbols` with all symbols that are called indirectly
   uint32_t SegmentCount = WasmObj->elements().size();
   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");

lld/trunk/wasm/SymbolTable.cpp

 
 // Get the signature for a given function symbol, either by looking
 // it up in function sections (for defined functions), of the imports section
 // (for imported functions).
 static const WasmSignature *getFunctionSig(const ObjFile &Obj,
                                            const WasmSymbol &Sym) {
   DEBUG(dbgs() << "getFunctionSig: " << Sym.Name << "\n");
   const WasmObjectFile *WasmObj = Obj.getWasmObj();
-  uint32_t FunctionType;
-  if (Obj.isImportedFunction(Sym.ElementIndex)) {
-    const WasmImport &Import = WasmObj->imports()[Sym.ImportIndex];
-    FunctionType = Import.SigIndex;
-  } else {
-    uint32_t FunctionIndex = Sym.ElementIndex - Obj.NumFunctionImports();
-    FunctionType = WasmObj->functionTypes()[FunctionIndex];
-  }
-  return &WasmObj->types()[FunctionType];
+  return &WasmObj->types()[Sym.FunctionType];
 }
 
 // Check the type of new symbol matches that of the symbol is replacing.
 // For functions this can also involve verifying that the signatures match.
 static void checkSymbolTypes(const Symbol &Existing, const InputFile &F,
                              const WasmSymbol &New,
                              const WasmSignature *NewSig) {
   if (Existing.isLazy())