Rollup merge of #121261 - Zalathar:pending-dups, r=oli-obk

coverage: Remove `pending_dups` from the span refiner

When extracting coverage spans from a function's MIR, we need to decide how to handle spans that are associated with more than one node (BCB) in the coverage control flow graph.

The existing code for managing those duplicate spans is very subtle and difficult to modify. But by eagerly deduplicating those extracted spans in a much simpler way, we can remove a massive chunk of complexity from the span refiner.

There is a tradeoff here, in that we no longer try to retain *all* nondominating BCBs that have the same span, only the last one in the (semi-arbitrary) dominance ordering. But in practice, this produces very little difference in our coverage tests, and the simplification is so significant that I think it's worthwhile.

``@rustbot`` label +A-code-coverage

Author: matthiaskrgr