[llvm] [TableGen, CodeGen, CHERI] Add support for the cPTR wildcard value type. (PR #158426)

[Owen Anderson via llvm-commits]

resistor wrote:

> We've been getting better about adding tablegen tests, particularly for globalisel, so [this](https://github.com/llvm/llvm-project/blob/main/llvm/test/TableGen/GlobalISelEmitter/OverloadedPtr.td) looks most directly applicable
> 
> I added a similar test recently [here](https://github.com/llvm/llvm-project/blob/c80d49590871c2060bbea758e09382bc5f9561c6/llvm/test/TableGen/RegClassByHwMode.td#L456)

Added a test case, PTAL

https://github.com/llvm/llvm-project/pull/158426