Refactor the formatter of clang-format.

Pull various parts of the UnwrappedLineFormatter into their own
abstractions. NFC.

There are two things left for subsequent changes (to keep this
reasonably small)
- the UnwrappedLineFormatter now has a bad name
- the UnwrappedLineFormatter::format function is still too large

llvm-svn: 236974
3 files changed