[clang-format] Add comment manipulation header

Summary:
Introduces a separate target for comment manipulation.
Currently, comment manipulation is in BreakableComment.cpp.
Towards implementing comment reflowing, we want to factor out the
comment-related functionality, so it can be reused.
Start simple by just moving out getLineCommentIndentPrefix.

Patch by Krasimir Georgiev!

Reviewers: djasper

Subscribers: klimek, beanz, mgorny, modocache

Differential Revision: https://reviews.llvm.org/D25725

llvm-svn: 284573