The compiler actually doesn't need any functional changes for this: Sema
does reification based on the tag indices of `std.builtin.Type` already!
So, no zig1.wasm update is necessary.
This change is necessary to disallow name clashes between fields and
decls on a type, which is a prerequisite of #9938.
The old heuristic of checking only for the number of fields has the
downside of classifying all opaque types, such as `std.c.FILE`, as
"namespaces" rather than "types".
