[nnc] Disable opaque pointers mode in LLVM backend to allow getPointerElementType (#88798)
As of LLVM 15 typed pointers are going away:
https://llvm.org/docs/OpaquePointers.html. Thus
`getPointerElementType` is no longer legal, since pointers are all
opaque. I don't totally remember why we use it so prolifically, or
whether there's an easy change to get rid of it, or whether we'd need
a significant refactor to carry around `Type`s alongside `Value`s.
But in any case, NNC is deprecated (see: TorchInductor) and will
hopefully be gone before LLVM 16 is a thing. For now, we can apply
the hack of turning off opaque pointer mode on the LLVMContext.
Differential Revision: [D41176215](https://our.internmc.facebook.com/intern/diff/D41176215)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88798
Approved by: https://github.com/desertfire
