Add llvm-mca to CODE_OWNERS.TXT

Please can we add a code owner entry for llvm-mca, which should probably be Andrea.

I wasn't sure if we need to go through a full nomination process for new tools like this (and llvm-exegesis?) or we can just add the primary contributor to the CODE_OWNERS.TXT list immediately.

Thanks, Simon.

Makes sense to me, please add it! Thanks again for contributing this work,

-Chris

Thanks!

Committed revision r330410.

Cheers

-Andrea