[Issue 21415] catch immutable exceptions breaks immutable

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21415

--- Comment #1 from Adam D. Ruppe <destructionator at gmail.com> ---
this might be a druntime bug instead of dmd. but i suspect it is both due to
too many void* in the interface layer.

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