This commits adds the following distinct integer types to std.zig.Ast:
- OptionalTokenIndex
- TokenOffset
- OptionalTokenOffset
- Node.OptionalIndex
- Node.Offset
- Node.OptionalOffset
The `Node.Index` type has also been converted to a distinct type while
`TokenIndex` remains unchanged.
`Ast.Node.Data` has also been changed to a (untagged) union to provide
safety checks.
Additionally, this commit streamlines the way unparseable files are handled, by giving them the AST of an empty file. This avoids bugs in the rest of the Autodoc logic trying to work with invalid ASTs.
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".
