replace std::variant with __variant in as_awaitable#1903

ericniebler wants to merge 3 commits intoNVIDIA:mainNVIDIA/stdexec:mainfrom

ericniebler:replace-use-of-std-variant-in-as-awaitableericniebler/stdexec:replace-use-of-std-variant-in-as-awaitableCopy head branch name to clipboard