[Issue 18561] postblit should allow writing const/immutable members just like constructors

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18561

--- Comment #3 from anonymous4 <dfj1esp02 at sneakemail.com> ---
This passes:
---
struct A
{
    int a;
    this(int b) const { a=b; }
}
int main()
{
    const A a;
    assert(a.a==0,"0");
    a.__ctor(1);
    assert(a.a==1,"1");
    return 0;
}
---

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