Add Clang ReleaseNotes that --autocomplete breaks backward compatibily

Summary:
--autocomplete flag now handles all the flags passed to shell, and this
implementation breaks backward compatibily before Clang 6.0.

Reviewers: teemperor, v.g.vassilev

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D44191

llvm-svn: 326889