Update the clang and clang-tools-extra code owners files

This also converts the Clang code owners file from a flat text file to
an RST file that is linked in to the rest of our documentation.

The RFC for this can be found at:
https://discourse.llvm.org/t/rfc-proposed-changes-to-clangs-code-ownership/

Differential Revision: https://reviews.llvm.org/D132550