<flat_set>: Make flat_(multi)set insertion exception-safe for non-standard sequence containers#5972

StephanTLavavej merged 2 commits intomicrosoft:feature/flat_mapmicrosoft/STL:feature/flat_mapfrom

frederick-vs-ja:fix-flat_set-insertionfrederick-vs-ja/STL:fix-flat_set-insertionCopy head branch name to clipboard