syntax: `Mod` records the span for inner contents.

this is useful when the module item and module contents are defined
from different files (like rustdoc). in most cases the original span
for the module item would be used; in other cases, the span for
module contents is available separately at the `inner` field.
rustdoc: External module item links to the module contents. Fixes #12926.

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.