This makes struct returns from functions work, but breaks
structs containing arrays, due to limitations in subsequent
transforms in spirv-opt. This is expected to be fixed soon.
Also, remove assumption that if something is opaque that it
must be in the UniformConstant storage class.
This allows function declarations to know all parameters will
be in the Function storage class.
For "s.m = t", a sampler member assigned a sampler, make t an alias
for s.m, and when s.m is flattened, it will flatten to the alias t.
Normally, assignments to samplers are disallowed.
