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

[Owen Anderson via llvm-commits]

resistor wrote:

> 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.

After checking in with the other CHERI folks, we prefer to keep the existing names unless there is a strong objection.

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