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

[Owen Anderson via llvm-commits]

resistor wrote:

> > Open question: should we omit the `c256` type? I believe it is purely historical at this point?
> 
> Yeah it is not needed upstream.

I've removed it.

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