This allows simplifying the call site and make_input by using a single
match instead of two levels of if's.

This allows re-entrant entering of the GlobalCtxt

We now only exit the GlobalCtxt when calling a callback and all the way
at the end when the GlobalCtxt is about to be destroyed.

There is no other query that may need to be called at that point anyway.

Several custom drivers are incorrectly calling queries.global_ctxt()
from inside of it, which causes some driver code to be skipped. As such
I would like to either remove it in the future or if custom drivers
still need it, change it to accept an &rustc_ast::Crate instead.