[llvm-dlltool] Clarify parameters, simplify ArgList usage. NFC.

[mstorsjo]

Summary

Add comments about unclear bool arguments to functions, switch to
hasArg instead of getLastArg for cases where we don't need to check
the argument's value.

[mati865]

LGTM

[jeremyd2019]

LGTM

llvm/lib/ToolDrivers/llvm-dlltool/DlltoolDriver.cpp

if (auto *Arg = Args.getLastArg(OPT_m))
     Machine = getEmulation(Arg->getValue());
 
   if (Machine == IMAGE_FILE_MACHINE_UNKNOWN) {
     llvm::errs() << "unknown target\n";
     return 1;
   }
 
   Expected<COFFModuleDefinition> Def =
-      parseCOFFModuleDefinition(*MB, Machine, true);
+      parseCOFFModuleDefinition(*MB, Machine, /*MingwDef=*/true);
 
   if (!Def) {
     llvm::errs() << "error parsing definition\n"
                  << errorToErrorCode(Def.takeError()).message();
     return 1;
   }
 
   // Do this after the parser because parseCOFFModuleDefinition sets OutputFile.
int llvm::dlltoolDriverMain(llvm::ArrayRef<const char *> ArgsArr) {
   // symbol decoration onto ExtName.
   for (COFFShortExport& E : Def->Exports) {
     if (!E.ExtName.empty()) {
       E.Name = E.ExtName;
       E.ExtName.clear();
     }
   }
 
-  if (Machine == IMAGE_FILE_MACHINE_I386 && Args.getLastArg(OPT_k)) {
+  if (Machine == IMAGE_FILE_MACHINE_I386 && Args.hasArg(OPT_k)) {
     for (COFFShortExport& E : Def->Exports) {
       if (!E.AliasTarget.empty() || (!E.Name.empty() && E.Name[0] == '?'))
         continue;
       E.SymbolName = E.Name;
       // Trim off the trailing decoration. Symbols will always have a
       // starting prefix here (either _ for cdecl/stdcall, @ for fastcall
       // or ? for C++ functions). Vectorcall functions won't have any
       // fixed prefix, but the function base name will still be at least
       // one char.
       E.Name = E.Name.substr(0, E.Name.find('@', 1));
       // By making sure E.SymbolName != E.Name for decorated symbols,
       // writeImportLibrary writes these symbols with the type
       // IMPORT_NAME_UNDECORATE.
     }
   }
 
-  if (!Path.empty() &&
-      writeImportLibrary(Def->OutputFile, Path, Def->Exports, Machine, true))
+  if (!Path.empty() && writeImportLibrary(Def->OutputFile, Path, Def->Exports,
+                                          Machine, /*MinGW=*/true))
     return 1;
   return 0;
 }