[lld][WebAssembly] Create optional symbols after handling --export/--undefined

[sbc100]

Summary

Handling of --export/--undefined can pull in lazy symbols which in turn
can pull in referenced to optional symbols. We need to delay the
creation of optional symbols until all possible references to them have
been created.

[ruiu]

LGTM

lld/trunk/test/wasm/Inputs/optional-symbol.ll

+target triple = "wasm32-unknown-unknown"
+
+@__dso_handle = external global i8*
+
+define i8** @get_optional() {
+  ret i8** @__dso_handle
+}

lld/trunk/test/wasm/export-optional-lazy.ll

+; Optional linker-synthetic symbols are only created if they are undefined
+; in the final output.
+; This test is for a regression where an explict --export of an lazy archive
+; symbol caused an undefined referece to an optional symbol to occur *after*
+; the optional symbols were created.
+
+; RUN: llc -filetype=obj %s -o %t.o
+; RUN: llc -filetype=obj %S/Inputs/optional-symbol.ll -o %t.a1.o
+; RUN: rm -f %t.a
+; RUN: llvm-ar rcs %t.a %t.a1.o
+; RUN: wasm-ld --export=get_optional %t.o %t.a -o %t.wasm
+; RUN: obj2yaml %t.wasm | FileCheck %s
+
+target triple = "wasm32-unknown-unknown"
+
+define void @_start() {
+entry:
+  ret void
+}
+
+; CHECK:      FunctionNames:
+; CHECK-NEXT:   - Index:           0
+; CHECK-NEXT:     Name:            _start
+; CHECK-NEXT:   - Index:           1
+; CHECK-NEXT:     Name:            get_optional

lld/trunk/wasm/Driver.cpp

void LinkerDriver::link(ArrayRef<const char *> argsArr) {
 
   // Add all files to the symbol table. This will add almost all
   // symbols that we need to the symbol table.
   for (InputFile *f : files)
     symtab->addFile(f);
   if (errorCount())
     return;
 
-  createOptionalSymbols();
-
   // Handle the `--undefined <sym>` options.
   for (auto *arg : args.filtered(OPT_undefined))
     handleUndefined(arg->getValue());
 
   // Handle the `--export <sym>` options
   // This works like --undefined but also exports the symbol if its found
   for (auto *arg : args.filtered(OPT_export))
     handleUndefined(arg->getValue());
 
   Symbol *entrySym = nullptr;
   if (!config->relocatable && !config->entry.empty()) {
     entrySym = handleUndefined(config->entry);
     if (entrySym && entrySym->isDefined())
       entrySym->forceExport = true;
     else
       error("entry symbol not defined (pass --no-entry to supress): " +
             config->entry);
   }
 
+  createOptionalSymbols();
+
   if (errorCount())
     return;
 
   // Create wrapped symbols for -wrap option.
   std::vector<WrappedSymbol> wrapped = addWrappedSymbols(args);
 
   // Do link-time optimization if given files are LLVM bitcode files.
   // This compiles bitcode files into real object files.