(Bloomberg) — A previously gloomy corner of the debt world has become the biggest winning trade in global financial markets, producing returns that few traders have seen in more than a decade.Most Read from BloombergClimate Migrants Stand to Overwhelm World’s MegacitiesVanderbilt Leases Struggling NYC Seminary for Campus ExpansionNew Rowhouses in London That Offer a Bridge to the 19th CenturyNJ Transit, Amtrak Trains Delayed After Derailment Near NYCWhere Cargo Bikes Are Freeing Cities From Pol