[libcxx] Add BOT_OWNERS.txt

Buildkite doesn't provide a way to list bot owners so currently
we are pinging people on Discord and Phabricator.

Which works ok until that person is on vacation. This file gives us
a place to list multiple people, or group contacts for each bot.

I've stuck to the CODE_OWNERS.txt format because there's no great
reason to change it.

Reviewed By: #libc, EricWF, ldionne

Differential Revision: https://reviews.llvm.org/D138445