[clang] [clang] Code owners -> Maintainers transition (PR #108997)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> FWIW, I plan to land these changes tomorrow morning.

I lied. :-D I just emailed both Manuel and Michael privately and I want them to have some time to consider and respond before landing the changes, but I did make anticipatory changes.

https://github.com/llvm/llvm-project/pull/108997