[Issue 18417] Make const and immutable postblit constructors illegal

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18417

--- Comment #6 from RazvanN <razvan.nitu1305 at gmail.com> ---
PR : https://github.com/dlang/dmd/pull/8032

The PR makes it illegal to declare a postblit const/immutable/shared, however,
it is still possible to make a postblit const/immutable/shared by appending a
qualifier to the struct:

immutable struct B
{
    this(this) {}
}

There might be code in the wild which does this but never actually uses the
postblit.

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