Add missing implementation for AtomicChange::replace(...)

Summary: Just realized the implementation is missing...

Reviewers: klimek

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D30735

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297289 91177308-0d34-0410-b5e6-96231b3b80d8