If two or more mappings cover exactly the same region, their relative order
will now be preserved from `get_expressions_and_counter_regions`, rather than
being disturbed by implementation details of an unstable sort.
The current order is: counter mappings, expression mappings, zero mappings.
(LLVM will also perform its own stable sort on these mappings, but that sort
only compares file ID, start location, and `RegionKind`.)
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