Extended Description

r312125, which introduced preprocessor indentation, shipped with a known
issue where "indentation of comments immediately before indented
preprocessor lines is toggled on each run". There's a second issue where trailing directives may be indented incorrectly. Both issues have reviews out:
https://reviews.llvm.org/D42036
https://reviews.llvm.org/D42035

I would prefer to do something for the 6.0 release. Some options include:

a) review and submit the patches above to trunk and then and merge to the release branch.
b) revert the feature for 6.0, keeping it on trunk.
c) document the known issues / mark the feature as experimental.