<chrono>: Updates two-digit year I/O for negative years per LWG-3831#6227

StephanTLavavej merged 2 commits intomicrosoft:mainmicrosoft/STL:mainfrom

MattStephanson:lwg-3831-neg-year-ioMattStephanson/STL:lwg-3831-neg-year-ioCopy head branch name to clipboard