[Issue 18670] compiler segfault if `new` on a union type with dip1000

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18670

--- Comment #3 from hsteoh at quickfur.ath.cx ---
P.S. Tested on git commit 2e21d0713babf760e4428d6404d733282ef85e8a, just to be
clear.

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