Integrate the SPIR-T qptr experiment.

[eddyb]

For more information, check out the SPIR-T PR, which has a much longer explanation:

• Introduce qptr ("quasi-pointer") type and associated lower->analyze->lift passes. spirt#24

The short version is goes something like this:

doing more (union/enum support, more flexible storage classes, PhysicalStorageBuffer, etc.)
with less (extremely untyped pointers, removing Rust-GPU's existing "storage class inference" pass)

Currently this PR allows testing replacing the old storage class inference with qptr passes, via:

RUSTGPU_CODEGEN_ARGS="--no-infer-storage-classes --spirt-passes=qptr"