incremental: Hide lang_items behind a query in the TyCtxt

[alexcrichton]

Right now this is calculated and passed into the TyCtxt, let's convert this to a query and then calculate it on-demand!

This'll also allow us to remove CrateStore::{lang_items, missing_lang_items} and convert them to queries as well.

[nikomatsakis]

cc me

[alexcrichton]

I've implemented this on a local branch, will post it soon