[Issue 19363] Manifest constant delegates are mutable

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19363

--- Comment #1 from lngns <contact at lngnslnvsk.net> ---
Actually, `const u = &(new C().foo);` fails even if C.foo does not mutate its
object and is annotated const.
So I would suggest failing only for non-const methods, though that may be a
bigger change.

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