[Bash-autocompletion] Check clang version in Bash

Summary:
Add check if user's clang version supports --autocomplete or not.
If not, we just autocomplete files.

Reviewers: ruiu, v.g.vassilev, teemperor

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D34607

llvm-svn: 306555