[flang][driver] Support parsing response files

Add support for reading response files in the flang driver. Response
files contain command line arguments and are used whenever a command
becomes longer than the shell/environment limit. Response files are
recognized via the special "@path/to/response/file.rsp" syntax, which
distinguishes them from other file inputs.

This patch hardcodes GNU tokenization, since we don't have a CL mode for
the driver. In the future we might want to add a --rsp-quoting command
line option, like clang has, to accommodate Windows platforms.

Differential Revision: https://reviews.llvm.org/D124846

flang/test/Driver/response-file.f90

@@ -0,0 +1,17 @@
+! Test that the driver can process response files.
+
+! RUN: echo "-DTEST" > %basename_t.rsp
+! RUN: %flang -E -cpp @%basename_t.rsp %s -o - | FileCheck %s
+! RUN: %flang_fc1 -E -cpp @%basename_t.rsp %s -o - | FileCheck %s
+! RUN: not %flang %basename_t.rsp %s -o /dev/null
+! RUN: not %flang_fc1 %basenamt_t.rsp %s -o /dev/null
+
+! CHECK-LABEL: program test
+! CHECK: end program
+
+#ifdef TEST
+program test
+end program
+#else
+We should have read the define from the response file.
+#endif

flang/tools/flang-driver/driver.cpp

@@ -25,6 +25,7 @@
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
 #include "llvm/Option/ArgList.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Host.h"
 #include "llvm/Support/InitLLVM.h"
 #include "llvm/Support/VirtualFileSystem.h"
@@ -71,6 +72,13 @@ static int executeFC1Tool(llvm::SmallVectorImpl<const char *> &argV) {
   return 1;
 }
 
+static void ExpandResponseFiles(
+    llvm::StringSaver &saver, llvm::SmallVectorImpl<const char *> &args) {
+  // We're defaulting to the GNU syntax, since we don't have a CL mode.
+  llvm::cl::TokenizerCallback tokenizer = &llvm::cl::TokenizeGNUCommandLine;
+  llvm::cl::ExpandResponseFiles(saver, tokenizer, args, /* MarkEOLs=*/false);
+}
+
 int main(int argc, const char **argv) {
 
   // Initialize variables to call the driver
@@ -80,6 +88,10 @@ int main(int argc, const char **argv) {
   clang::driver::ParsedClangName targetandMode("flang", "--driver-mode=flang");
   std::string driverPath = getExecutablePath(args[0]);
 
+  llvm::BumpPtrAllocator a;
+  llvm::StringSaver saver(a);
+  ExpandResponseFiles(saver, args);
+
   // Check if flang-new is in the frontend mode
   auto firstArg = std::find_if(
       args.begin() + 1, args.end(), [](const char *a) { return a != nullptr; });