[Bug fortran/56226] Add support for DEC UNION and MAP extensions

[fritzoreese at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56226

--- Comment #13 from Fritz Reese <fritzoreese at gmail dot com> ---
I actually have a fairly in-depth set of testcases as part of the patch (based
on lots of legacy code). More are of course welcome, but check out what I've
already added as a starting point.