[objcmt] If the frontend option is frontend::MigrateSource then we don't need to create
the arcmt wrappers.
[objcmt] Transfer the objcmt flags from the driver to cc1 invocation and
instantiate ObjCMigrateASTConsumer with the specific options that were enabled.
[objcmt] If no objcmt options were specified, default to enable literals+subscripting.