[Issue 21963] New: importC: Support declaring union types

[d-bugmail at puremagic.com]

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

          Issue ID: 21963
           Summary: importC: Support declaring union types
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: critical
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: ibuclaw at gdcproject.org

Unions are rejected by the compiler, and yet structs are not.
---
union a_union
{
    int iv;
    float fv;
};

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