[mlir][GPUToLLVM] Add support for emitting opaque pointers

Part of https://discourse.llvm.org/t/rfc-switching-the-llvm-dialect-and-dialect-lowerings-to-opaque-pointers/68179

This patch adds the new pass option `use-opaque-pointers` to the GPU to LLVM lowerings (including ROCD and NVVM) and adapts the code to support using opaque pointers in addition to typed pointers.
The required changes mostly boil down to avoiding `getElementType` and specifying base types in GEP and Alloca.

In the future opaque pointers will be the only supported model, hence tests have been ported to using opaque pointers by default. Additional regression tests for typed-pointers have been added to avoid breaking existing clients.

Note: This does not yet port the `GpuToVulkan` passes.

Differential Revision: https://reviews.llvm.org/D144448