[LLD] [MinGW] Implement --help for the MinGW driver

[ruiu]

LGTM

MinGW/Driver.cpp

 namespace {
 class MinGWOptTable : public opt::OptTable {
 public:
   MinGWOptTable() : OptTable(InfoTable, false) {}
   opt::InputArgList parse(ArrayRef<const char *> Argv);
 };
 } // namespace
 
+static void printHelp(const char *Argv0) {
+  MinGWOptTable().PrintHelp(
+      outs(), (std::string(Argv0) + " [options] file...").c_str(), "lld",
+      false /*ShowHidden*/, true /*ShowAllAliases*/);
+  outs() << "\n";
+}
+
 opt::InputArgList MinGWOptTable::parse(ArrayRef<const char *> Argv) {
   unsigned MissingIndex;
   unsigned MissingCount;
 
   SmallVector<const char *, 256> Vec(Argv.data(), Argv.data() + Argv.size());
   opt::InputArgList Args = this->ParseArgs(Vec, MissingIndex, MissingCount);
 
   if (MissingCount)
     fatal(StringRef(Args.getArgString(MissingIndex)) + ": missing argument");
   for (auto *Arg : Args.filtered(OPT_UNKNOWN))
     fatal("unknown argument: " + Arg->getSpelling());
-  if (!Args.hasArg(OPT_INPUT) && !Args.hasArg(OPT_l))
-    fatal("no input files");
   return Args;
 }
 
 // Find a file by concatenating given paths.
 static Optional<std::string> findFile(StringRef Path1, const Twine &Path2) {
   SmallString<128> S;
   sys::path::append(S, Path1, Path2);
   if (sys::fs::exists(S))

 }
 
 // Convert Unix-ish command line arguments to Windows-ish ones and
 // then call coff::link.
 bool mingw::link(ArrayRef<const char *> ArgsArr, raw_ostream &Diag) {
   MinGWOptTable Parser;
   opt::InputArgList Args = Parser.parse(ArgsArr.slice(1));
 
+  if (Args.hasArg(OPT_help)) {
+    printHelp(ArgsArr[0]);
+    return true;
+  }
+
+  if (!Args.hasArg(OPT_INPUT) && !Args.hasArg(OPT_l))
+    fatal("no input files");
+
   std::vector<std::string> LinkArgs;
   auto Add = [&](const Twine &S) { LinkArgs.push_back(S.str()); };
 
   Add("lld-link");
   Add("-lldmingw");
 
   if (auto *A = Args.getLastArg(OPT_entry)) {
     StringRef S = A->getValue();

MinGW/Options.td

 def dynamicbase: F<"dynamicbase">, HelpText<"Enable ASLR">;
 def entry: S<"entry">, MetaVarName<"<entry>">,
   HelpText<"Name of entry point symbol">;
 def exclude_all_symbols: F<"exclude-all-symbols">,
     HelpText<"Don't automatically export any symbols">;
 def export_all_symbols: F<"export-all-symbols">,
     HelpText<"Export all symbols even if a def file or dllexport attributes are used">;
 def gc_sections: F<"gc-sections">, HelpText<"Remove unused sections">;
+def help: F<"help">, HelpText<"Print option help">;
 def icf: J<"icf=">, HelpText<"Identical code folding">;
 def image_base: S<"image-base">, HelpText<"Base address of the program">;
 def insert_timestamp: F<"insert-timestamp">,
     HelpText<"Include PE header timestamp">;
 def kill_at: F<"kill-at">, HelpText<"Remove @n from exported symbols">;
 def l: JoinedOrSeparate<["-"], "l">, MetaVarName<"<libName>">,
   HelpText<"Root name of library to use">;
 def m: JoinedOrSeparate<["-"], "m">, HelpText<"Set target emulation">;