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

[David Spickett via cfe-commits]

DavidSpickett wrote:

@tobiasgrosser 

(because for some reason I can't add you as a reviewer directly)

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