[Issue 18100] crt_constructor not allow to init immutable

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18100

--- Comment #1 from changlon <changlon at gmail.com> ---
crt_constructor can be call from anywhere. If so it is not able to init
immutable.

But if it it not able to init immutable,  it will be much less useful from
BetterC (compare to static this).

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