[mac/lld] Add support for response files

[thakis]

Summary

ld64 learned about them in Xcode 12, so we should too.

[int3]

lgtm

lld/MachO/DriverUtils.cpp

 //===- DriverUtils.cpp ----------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "Driver.h"
 #include "InputFiles.h"
 
 #include "lld/Common/ErrorHandler.h"
 #include "lld/Common/Memory.h"
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Option/Option.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Path.h"
 #include "llvm/TextAPI/MachO/TextAPIReader.h"
 
 using namespace llvm;
 using namespace llvm::MachO;
 using namespace llvm::opt;
 using namespace llvm::sys;
 using namespace lld;

 }
 
 opt::InputArgList MachOOptTable::parse(ArrayRef<const char *> argv) {
   // Make InputArgList from string vectors.
   unsigned missingIndex;
   unsigned missingCount;
   SmallVector<const char *, 256> vec(argv.data(), argv.data() + argv.size());
 
+  // Expand response files (arguments in the form of @<filename>)
+  // and then parse the argument again.
+  cl::ExpandResponseFiles(saver, cl::TokenizeGNUCommandLine, vec);
   opt::InputArgList args = ParseArgs(vec, missingIndex, missingCount);
 
   if (missingCount)
     error(Twine(args.getArgString(missingIndex)) + ": missing argument");
 
   handleColorDiagnostics(args);
 
   for (opt::Arg *arg : args.filtered(OPT_UNKNOWN)) {

lld/test/MachO/responsefile.test

+# RUN: echo --help > %t.rsp
+
+# RUN: %lld @%t.rsp | FileCheck %s
+CHECK: OVERVIEW: LLVM Linker