Function create_mappings

fn create_mappings<'tcx>(
    tcx: TyCtxt<'tcx>,
    hir_info: &ExtractedHirInfo,
    extracted_mappings: &ExtractedMappings,
    coverage_counters: &CoverageCounters,
) -> Vec<Mapping>

For each coverage span extracted from MIR, create a corresponding mapping.

Precondition: All BCBs corresponding to those spans have been given coverage counters.