Summary:
OS X seems to use older bash version which doesn't suport
_init_completion and compopt, so add support for this.

Reviewers: ruiu, v.g.vassilev, teemperor

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D34924

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@306962 91177308-0d34-0410-b5e6-96231b3b80d8