[Issue 19928] disallow modification of immutable in constructor after calling base ctor

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19928

--- Comment #5 from Andrei Alexandrescu <andrei at erdani.com> ---
So a solution to the super() issue would be to require the immutable fields are
assigned before calling super(). It is odd but it works.

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