When producing diagnostics about missing attributes or bodies it's
necessary to have a range representing the place where the missing thing
might be inserted.
There's not always a single reasonable value for this, so some liberty
can be taken about what exactly is returned as long as it's somewhere
the user can relate back to the construct producing the error.
When using the parser to do static analysis and editor integrations, it's
still useful to be able to get the incomplete AST resulting from a parse
error.
Callers that intend to use the returned value to take real actions (as
opposed to just analysis) must check diags.HasError() to determine if
the returned file can be considered valid.
