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

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM

For the VT name itself, an option to consider may be `cap128` instead of `c128`? I think that may give a stronger hint on what this refers to while still retaining brevity. No strong opinion though.

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