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

[Nikita Popov via cfe-commits]

nikic wrote:

For openmp I'm not really sure, maybe @jdoerfert or @jpeyton52 can advise.

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