Auto merge of #31304 - nikomatsakis:incr-comp-read-from-hir-map, r=michaelwoerister

This change also modifies the dep graph infrastructure to keep track of the number of active tasks, so that even if we are not building the full dep-graph, we still get assertions when there is no active task and one does something that would add a read/write edge. This is particularly helpful since, if the assertions are *not* active, you wind up with the error happening in the message processing thread, which is too late to know the correct backtrace.

~~Before landing, I need to do some performance measurements. Those are underway.~~

See measurements below. No real effect on time.

r? @michaelwoerister

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