[llvm] [C API] Add getters for Target Extension Types to C API (PR #71291)

[Benji Smith via llvm-commits]

Benjins wrote:

> Looking at the other methods, I think it would also help to have a get I'th type/integer parameter method too in the API.
> These types should be SmallVector<LLVMTypeRef, 4> and SmallVector<unsigned, 4>, respectively...

These should both be taken care of after the couple commits I pushed just now

https://github.com/llvm/llvm-project/pull/71291