Summary:
An AtomicChange is used to create and group a set of source edits, e.g.
replacements or header insertions. Edits in an AtomicChange should be related,
e.g. replacements for the same type reference and the corresponding header
insertion/deletion.

An AtomicChange is uniquely identified by a key position and will either be
fully applied or not applied at all. The key position should be the location
of the key syntactical element that is being changed, e.g. the call to a
refactored method.

Next step: add a tool that applies AtomicChange.

Reviewers: klimek, djasper

Reviewed By: klimek

Subscribers: alexshap, cfe-commits, djasper, mgorny

Differential Revision: https://reviews.llvm.org/D27054

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@296616 91177308-0d34-0410-b5e6-96231b3b80d8