#[ast] attribute is not meaningful unless the type it's on is in a file which is processed by AST codegen.

Make macro raise an error/panic if the file path needs to be added to AST codegen, with an error that says what to do. This will help people do the right thing.

• Error on invalid arguments to generate_derive (#4766)
• Error if fundamental type names such as Span and primitives don't meet our expectations
• Error on usage of #[ast] attribute on types outside of ast_codegen's view