Add a unified Clang JLL using platform augmentation.

[maleadt]

Probably needs to be a new JLL because the previous one uses packages versions for compatibility (although unified might not be the best suffix).

This fixes the issue where Clang_jll.jl currently sometimes loads the wrong LLVM library, as reported by @jpsamaroo.

[vchuravy]

Do we want to do this for LLVM@14? Maybe cleaner then adding a new package? and I also want to add a separate LLD_jll