[Issue 21963] importC: Support declaring union types

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21963

--- Comment #2 from Iain Buclaw <ibuclaw at gdcproject.org> ---
Committed as
https://github.com/dlang/dmd/commit/2453350ae801d689d25f2b89d3a63897091fdfbb

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