[llvm] [C API] Support new ptrauth constant type (PR #93909)

[Anton Korobeynikov via llvm-commits]

asl wrote:

> @ahmedbougacha @asl Is there any risk that something about `ptrauth` constants will change in the near future (in ways that would affect stability of the C API)?

Given the stability guarantees of C API, I would wait until we will have end-to-end implementation from frontend to backend just in case.

https://github.com/llvm/llvm-project/pull/93909