Remove 'RUN: false' from Driver/response-file.c

Summary:
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.

Reviewers: bkramer

Reviewed By: bkramer

Subscribers: cfe-commits, rsmith

Differential Revision: https://reviews.llvm.org/D28941

llvm-svn: 292600