Revert "[Driver] Avoid using response files for SourceKit"

This reverts commit 6eb76df.

Author: hamishknight

lib/Driver/ToolChain.cpp

@@ -72,11 +72,6 @@ std::optional<Job::ResponseFileInfo>
 ToolChain::getResponseFileInfo(const Compilation &C, const char *executablePath,
                                const ToolChain::InvocationInfo &invocationInfo,
                                const ToolChain::JobContext &context) const {
-  // Never use a response file if this is a dummy driver for SourceKit, we
-  // just want the frontend arguments.
-  if (getDriver().isDummyDriverForFrontendInvocation())
-    return std::nullopt;
-
   const bool forceResponseFiles =
       C.getArgs().hasArg(options::OPT_driver_force_response_files);
   assert((invocationInfo.allowsResponseFiles || !forceResponseFiles) &&