It seems that rL292518 introduced a RUN: false, but the continuation rL292545
forgot to remove it back.
This has flown under the radar, because it's a long test and doesn't get
executed by default during sanity testing.
To test:
$ cd llvm_build
$ ./bin/llvm-lit --param run_long_tests=true tools/clang/test/Driver/response-file.c
@rsmith: have a look if this change is OK please.