This code was calling `sort_unstable_by`, but failed to impose a total order on
the initial spans. That resulted in unpredictable handling of closure spans,
producing inconsistencies in the coverage maps and in user-visible coverage
reports.
This patch fixes the problem by always sorting closure spans before
otherwise-identical non-closure spans, and also switches to a stable sort in
case the ordering is still not total.
The output of these tests is too complicated to comfortably verify by hand, but
we can still use them to observe changes to the underlying mappings produced by
codegen/LLVM.
If these tests fail due to non-coverage changes (e.g. in HIR-to-MIR lowering or
MIR optimizations), it should usually be OK to just `--bless` them, as long as
the `run-coverage` test suite still works.
