Added `applyAtomicChanges` function.

This re-commits r298913.
o See thread http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20170327/189084.html
o Tested with -DLLVM_ENABLE_LOCAL_SUBMODULE_VISIBILITY=0.

Summary: ... which applies a set of `AtomicChange`s on code.

Reviewers: klimek, djasper

Reviewed By: djasper

Subscribers: arphaman, mgorny, chapuni, cfe-commits

Differential Revision: https://reviews.llvm.org/D30777

llvm-svn: 309548