[clangd] Revamp textDocument/onTypeFormatting.

Summary:
The existing implementation (which triggers on }) is fairly simple and
has flaws:
 - doesn't trigger frequently/regularly enough (particularly in editors that type the }
 for you)
 - often reformats too much code around the edit
 - has jarring cases that I don't have clear ideas for fixing

This implementation is designed to trigger on newline, which feels to me more
intuitive than } or ;.
It does have allow for reformatting after other characters - it has a
basic behavior and a model for adding specialized behavior for
particular characters. But at least initially I'd stick to advertising
\n in the capabilities.

This also handles comment splitting: when you insert a line break inside
a line comment, it will make the new line into an aligned line comment.

Working on tests, but want people to patch it in and try it - it's hard to
see if "feel" is right purely by looking at a test.

Reviewers: ilya-biryukov, hokein

Subscribers: mgorny, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60605

llvm-svn: 362939