Don't serialize ExpnData for foreign crates
When we encode an ExpnId into the crate metadata, we write out the
CrateNum of the crate that 'owns' the corresponding `ExpnData`, which
is later used to decode the `ExpnData` from its owning crate.
However, we current serialize the `ExpnData` for all `ExpnIds` that we
serialize, even if the `ExpnData` was already serialized into a foreign
crate. This commit skips encoding this kind of `ExpnData`, which should
hopefully speed up metadata encoding and reduce the total metadata size.