the basic strategy is to distinguish `mod foo;` from `mod foo {...}`
by checking if the span for the module item and module contents is
in different files. if it's the case, we prefer module contents.
it is technically possible to fix#12926 without changing the AST,
probably by checking the individual items' span. this is not without
a problem though, since it is possible that some items inside
`mod foo {...}` may have originated from other file (e.g. `include!`).
therefore it is better to record both spans explicitly.
c8a29c4c59