[Issue 18604] in parameter storage class should be deprecated

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18604

--- Comment #7 from anonymous4 <dfj1esp02 at sneakemail.com> ---
Another possibility is -transition=in flag that will deprecate `in` on demand,
and after people remove usage of `in` storage class from their code, it can
start enforce `scope const` meaning.

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