This continues to prevent writing output buffers (out from a function),
but fixes the problem where the copy-in/out was not getting done.
Making everything work will require knowing both in/out-ness and bufferness,
but these are currently mutually exclusive, because both are storage
qualifiers.
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.
Byte address buffers were failing to detect that they were byte address
buffers when used as fn parameters.
Note: this detection is a little awkward, and could be simplified if
it was easy to obtain the declared builtin type for an object.
