Restructure how we break tokens.

This fixes some bugs in the reflowing logic and splits out the concerns
of reflowing from BreakableToken.

Things to do after this patch:
- Refactor the breakProtrudingToken function possibly into a class, so we
  can split it up into methods that operate on the common state.
- Optimize whitespace compression when reflowing by using the next possible
  split point instead of the latest possible split point.
- Retry different strategies for reflowing (strictly staying below the
  column limit vs. allowing excess characters if possible).

Differential Revision: https://reviews.llvm.org/D40310

llvm-svn: 319314