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.
auto merge of #13791 : lifthrasiir/rust/mod-inner-span, r=huonw

This PR is primarily motivated by (and fixes) #12926.

We currently only have a span for the individual item itself and not for the referred contents. This normally does not cause a problem since both are located in the same file; it *is* possible that the contained statement or item is located in the other file (the syntax extension can do that), but even in that case the syntax extension should be located in the same file as the item. The module item (i.e. `mod foo;`) is the only exception here, and thus warrants a special treatment.

Rustdoc would now 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'd prefer module contents over module item. There are alternative strategies, but as noted above we will have some corner cases if we don't record the contents span explicitly.