incr.comp.: Make cross-crate tracking for incr. comp. opt-in.
The current implementation of cross-crate dependency tracking can cause quite long compile times and high memory usage for some crates (see #39208 for example). This PR therefore makes that part of dependency tracking optional. Incremental compilation still works, it will only have very coarse dep-tracking for upstream crates.
r? @nikomatsakis
Setup two tasks, one of which only processes the signatures, in order to
isolate the typeck entries for signatures from those for bodies.
Fixes#36078Fixes#37720
Before, when we created an AssociatedItem for impl item X, we would read
the impl item itself. Now we instead load up the impl I that contains X
and read the data from the `ImplItemRef` for X; actually, we do it for
all impl items in I pre-emptively.
This kills the last source of edges between a method X and a call to a
method Y defined in the same impl.
Fixes#37121
