Forbid unsized structs as final members of structs. (#1443)

WGSL says:

> - The last member of the structure type defining the store type for a variable
>   ... may be a runtime-sized array.
>
> - A runtime-sized array must not be used as the store type or contained within
>   a store type in any other cases.

Thus, a struct whose final member is a struct whose final member is a
runtime-sized array is verboten.