<type_traits>: Make is_nothrow_convertible a class template and ADL-proof#4318

StephanTLavavej merged 3 commits intomicrosoft:mainmicrosoft/STL:mainfrom

frederick-vs-ja:is_nothrow_convertible-fixfrederick-vs-ja/STL:is_nothrow_convertible-fixCopy head branch name to clipboard