[clang-format] Implement comment reflowing.

Summary:
This presents a version of the comment reflowing with less mutable state inside
the comment breakable token subclasses. The state has been pushed into the
driving breakProtrudingToken method. For this, the API of BreakableToken is enriched
by the methods getSplitBefore and getLineLengthAfterSplitBefore.

Reviewers: klimek

Reviewed By: klimek

Subscribers: djasper, klimek, mgorny, cfe-commits, ioeric

Differential Revision: https://reviews.llvm.org/D28764

llvm-svn: 293055