[llvm] [CodeGen, CHERI] Add capability types to MVT. (PR #156616)

[Alexander Richardson via llvm-commits]

https://github.com/arichardson commented:

I would add @davidchisnall and @jrtc27 to `Co-authored-by:` but otherwise this looks good to me once c256 is gone.

Question to other LLVM maintainers, is the "Capability" name okay here, or does it need to be something more explicit like "CheriCapability"?

https://github.com/llvm/llvm-project/pull/156616