[llvm-rc] Fix the reference to the option for disabling preprocessing in a message

[mstorsjo]

Summary

This was the original option name from the first iteration of the patch
that added the feature, but during review, a different name was suggested
and preferred - but the reference in the helpful message was missed.

[aganea]

LG.

llvm/tools/llvm-rc/llvm-rc.cpp

struct RcOptions {
   std::vector<std::string> PreprocessCmd;
   std::vector<std::string> PreprocessArgs;
 
   std::string InputFile;
   Format InputFormat = Rc;
   std::string OutputFile;
   Format OutputFormat = Res;
 
+  bool IsWindres = false;
   bool BeVerbose = false;
   WriterParams Params;
   bool AppendNull = false;
   bool IsDryRun = false;
   // Set the default language; choose en-US arbitrarily.
   unsigned LangId = (/*PrimaryLangId*/ 0x09) | (/*SubLangId*/ 0x01 << 10);
 };
 
 bool preprocess(StringRef Src, StringRef Dst, const RcOptions &Opts,
                 const char *Argv0) {
   std::string Clang;
   if (Opts.PrintCmdAndExit || !Opts.PreprocessCmd.empty()) {
     Clang = "clang";
   } else {
     ErrorOr<std::string> ClangOrErr = findClang(Argv0, Opts.Triple);
     if (ClangOrErr) {
       Clang = *ClangOrErr;
     } else {
       errs() << "llvm-rc: Unable to find clang, skipping preprocessing."
              << "\n";
-      errs() << "Pass -no-cpp to disable preprocessing. This will be an error "
-                "in the future."
-             << "\n";
+      StringRef OptionName =
+          Opts.IsWindres ? "--no-preprocess" : "-no-preprocess";
+      errs()
+          << "Pass " << OptionName
+          << " to disable preprocessing. This will be an error in the future."
       return false;
     }
   }
 
   SmallVector<StringRef, 8> Args = {
       Clang, "--driver-mode=gcc", "-target", Opts.Triple, "-E",
       "-xc", "-DRC_INVOKED"};
   if (!Opts.PreprocessCmd.empty()) {

 RcOptions parseWindresOptions(ArrayRef<const char *> ArgsArr,
                               ArrayRef<const char *> InputArgsArray,
                               std::string Prefix) {
   WindresOptTable T;
   RcOptions Opts;
   unsigned MAI, MAC;
   opt::InputArgList InputArgs = T.ParseArgs(ArgsArr, MAI, MAC);
 
+  Opts.IsWindres = true;
+
   // The tool prints nothing when invoked with no command-line arguments.
   if (InputArgs.hasArg(WINDRES_help)) {
     T.printHelp(outs(), "windres [options] file...",
                 "LLVM windres (GNU windres compatible)", false, true);
     exit(0);
   }
 
   if (InputArgs.hasArg(WINDRES_version)) {