Each source region has a start and end location. Report an error when
the end location does not precede the begin location.
The old lineExecutionCounts.covmapping test actually had a buggy source
region in it. This commit introduces a regenerated copy of the coverage
and moves the old copy to malformedRegions.covmapping, for a test.
This is needed to unblock D36813, since the proposed segment builder
changes assert-fail when given bogus input.