additional de-stringify pass for unions

additional de-stringify pass for unions

the change in c3cfee5b00a40790c18d took out
a pass for de-stringify that broke some un-tested cases
for Optional with future annotations mode.   Adding tests
for this revealed that this was a subset of
a more general case where Union is presented
with ForwardRefs inside of it matching up within the type
map, which wasn't working before either, fixed that as well with
an additional de-stringify for elements within the Union.

Fixes: #8478
Change-Id: I8804cf6c67f14d10804584e1cddd2cfaa2376654