spirv: Handle OpTypeForwardPointer

spirv: Handle OpTypeForwardPointer

We handle forward declarations by creating the pointer type with it's
storage type based on storage class and just waiting to fill out the
actual deref type until we get the OpTypePointer.  Because any
composites using the forward declared type only care about the storage
type (i.e. uint64_t, uvec2, etc.) when creating their glsl_type, this
works fine and we can defer the actual deref_type as far as we need.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Karol Herbst <kherbst@redhat.com>