[Bash-autocompletion] Add comment to test so that it is easier to fix

Summary:
clang/test/Driver/autocomplete.c is a test for --autocomplete, and this
test might break if people add/modify flags or HelpText. So I've add
comment for future developers so that they can fix this file according
to the change they had made.

Reviewers: v.g.vassilev, teemperor, ruiu

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D36209

llvm-svn: 309794