Revert "upgrade TaggedUnion"
Revert "upgrade TaggedUnion"

This reverts commit f4c5c64492fe590e780f9c829112d0e8d9bf0cd2.

A-Manning/FStar,FStarLang/FStar,FStarLang/FStar,A-Manning/FStar,A-Manning/FStar,FStarLang/FStar