[Driver/SourceKit] Handle filelist driver args in getSingleFrontendInvocationFromDriverArguments

getSingleFrontendInvocationFromDriverArguments is set up to never produce file
lists in the output frontend arguments, but since the driver accepts file lists
as input arguments, this should too.

lib/Driver/FrontendUtil.cpp

@@ -19,6 +19,8 @@
 #include "swift/Driver/Job.h"
 #include "swift/Driver/ToolChain.h"
 #include "llvm/Option/ArgList.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/StringSaver.h"
 
 using namespace swift;
 using namespace swift::driver;
@@ -46,6 +48,16 @@ bool swift::driver::getSingleFrontendInvocationFromDriverArguments(
   Args.push_back("-driver-filelist-threshold");
   Args.push_back(neverThreshold.c_str());
 
+  // Expand any file list args.
+  llvm::BumpPtrAllocator Allocator;
+  llvm::StringSaver Saver(Allocator);
+  llvm::cl::ExpandResponseFiles(
+      Saver,
+      llvm::Triple(llvm::sys::getProcessTriple()).isOSWindows()
+          ? llvm::cl::TokenizeWindowsCommandLine
+          : llvm::cl::TokenizeGNUCommandLine,
+      Args);
+
   // Force the driver into batch mode by specifying "swiftc" as the name.
   Driver TheDriver("swiftc", "swiftc", Args, Diags);