[lld][ELF] Check for errors when the response file doesn't exist.

[oontvoo]

Summary

The ExpandResponseFile function could return an error when it failed to expand the file (because it doens't exist or it's not readable, etc).
The caller should check for that rather than ignoring the return value then later on trying to load the @file.txt as an input file

similar change has also been made to lld-macho

lld/ELF/DriverUtils.cpp

opt::InputArgList ELFOptTable::parse(ArrayRef<const char *> argv) {
 
   // We need to get the quoting style for response files before parsing all
   // options so we parse here before and ignore all the options but
   // --rsp-quoting.
   opt::InputArgList args = this->ParseArgs(vec, missingIndex, missingCount);
 
   // Expand response files (arguments in the form of @<filename>)
   // and then parse the argument again.
-  cl::ExpandResponseFiles(saver, getQuotingStyle(args), vec);
+  checkError(
+      cl::ExpandResponseFilesWithError(saver, getQuotingStyle(args), vec));
+
   concatLTOPluginOptions(vec);
   args = this->ParseArgs(vec, missingIndex, missingCount);
 
   handleColorDiagnostics(args);
   if (missingCount)
     error(Twine(args.getArgString(missingIndex)) + ": missing argument");
 
   for (opt::Arg *arg : args.filtered(OPT_UNKNOWN)) {