[Issue 23841] New: isZeroInit does not take into account unions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23841

          Issue ID: 23841
           Summary: isZeroInit does not take into account unions
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P4
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: dkorpel at live.nl

```
union U 
{
    float x = 0;
    float y;
}

static assert(__traits(isZeroInit, U)); // fails
```

It also wastes bytes in the binary because it emits `U.init` in .rodata instead
of .bss.

Workaround: add `=void` to void initialize the other union members.

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