[Issue 22999] New: no switch fallthrough error with multi-valued case

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22999

          Issue ID: 22999
           Summary: no switch fallthrough error with multi-valued case
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: dkorpel at live.nl

I forgot `break`, but the compiler didn't raise an error:
```
void main()
{
    int i;
    switch (0) {
        case 0, 1: i = 20;
        default: assert(0);
    }
}
```
Looks similar to issue 11653

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