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

[David Spickett via cfe-commits]

DavidSpickett wrote:

Libcxx part pushed as https://github.com/llvm/llvm-project/commit/1bdb0a408f1e2e9f6d5f43b23b4f87f87b3950ab.

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