[Issue 24332] Improve downcast to final classes

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24332

--- Comment #1 from Walter Bright <bugzilla at digitalmars.com> ---
See https://github.com/dlang/dmd/pull/16027 for partial implementation (without
inlining)

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