[Issue 21415] catch immutable exceptions breaks immutable

[d-bugmail at puremagic.com]

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

--- Comment #3 from Adam D. Ruppe <destructionator at gmail.com> ---
Yeah, the immutability being lost destroyed what I was hoping to do with this,
so definitely the bigger problem. But it being lost either direction is
problematic.

Really a "catch-all" would be `catch(const Throwable)`. Const and immutable
objects should NOT be caught by a catch mutable. I realize nobody thinks about
this but we probably should start.

An immutable exception can be used without allocation concerns.

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