clang-tools-extra code owners

[Aaron Ballman via cfe-commits]

I happened to notice that CODE_OWNERS.TXT for clang-tools-extra is
rather sparse. These tools are starting to get more attention from the
community, and I am wondering whether it would make sense to populate
that somewhat? If so, I would nominate:

Alexander Kornienko for clang-tidy
Peter Collingbourne for clang-query
Manuel Klimek for clang-rename, and clang-tools-extra as a whole

I'm basing these nominations on who is actively maintaining the tools already.

~Aaron