[libclc] [llvm] [polly] [llvm] Move sub-project lead maintainers into their own Maintainers.md files (PR #118309)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

@tobiasgrosser and me already had talked about this at an LLVM conference. I didn't push the maintainer change yet because I am myself somewhat slow with reviews, but I think it makes sense.

https://github.com/llvm/llvm-project/pull/118309