Propagate changes through no-op transforms

Currently, changes made by previous transforms are not kept if a transform
doesn't make any changes itself to a given file. Now file states are propagated
properly through transforms that don't make changes.

Fixes: PR15281
Author: Jack Yang <jack.yang@intel.com>
Reviewer: klimek



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@175288 91177308-0d34-0410-b5e6-96231b3b80d8