Add a utility for converting between different types of remarks

This adds llvm-remarkutil. This is intended to be a general tool for doing stuff
with/to remark files.

This patch gives it the following powers:

* `bitstream2yaml` - To convert bitstream remarks to YAML
* `yaml2bitstream` - To convert YAML remarks to bitstream remarks

These are both implemented as subcommands, like

`llvm-remarkutil bitstream2yaml <input_file> -o -`

I ran into an issue where I had some bitstream remarks coming from CI, and I
wanted to be able to do stuff with them (e.g. visualize them). But then I
noticed we didn't have any tooling for doing that, so I decided to write this
thing.

Being able to output YAML as a start seemed like a good idea, since it
would allow people to reuse any tooling they may have written based around YAML
remarks.

Hopefully it can grow into a more featureful remark utility. :)

Currently there are is an outstanding performance issue (see the TODO) with
the bitstream2yaml case. I decided that I'd keep the tool small to start with
and have the yaml2bitstream and bitstream2yaml cases be symmetric.

Also I moved the remarks documentation to its own header because it seems
a little out of place with "basic commands" and "developer tools"; it's
really kind of its own thing.

Differential Revision: https://reviews.llvm.org/D133646