[parallel-queries] Refactor layout-depth tracking so it does not need mutable state in the GlobalCtxt

[michaelwoerister]

As described here, checking for too deep recursion when computing data layouts should not need to rely on mutable state in the GlobalCtxt.

@Zoxc has implemented one possible solution here: cb51381

Maybe there is an even simpler one that does not require explicitly entering a new tcx.

cc @eddyb & @nikomatsakis

[nikomatsakis]

I would think we could just walk the stack and check how many "layout" queries are on it -- maybe without anything in between