[Issue 18628] @disable this(this) erroneously adds `__postblit` member

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18628

--- Comment #4 from Andrei Alexandrescu <andrei at erdani.com> ---
The problem with hasElaborateCopyConstructor is the binary expectation:

* is that true? Then there's work involved to copy the object
* is that false? Then the object can be copied with memcpy

This does not account for the disabled case. So I'm not sure what to do here
aside from defining a different trait e.g. hasDisabledCopyConstructor.

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