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

[Owen Anderson via llvm-commits]

resistor wrote:

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

Good question. @nikic what's your view on nomenclature here?

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