Auto merge of #42055 - michaelwoerister:reenable-incremental-cc, r=nikomatsakis
Enable cross-crate incremental compilation by default.
Now that direct metadata hashing has been implemented for a while and we haven't seen any problems with it over at [rust-icci](https://travis-ci.org/rust-icci/), let's re-enable cross crate support for incremental compilation again.
r? @nikomatsakis