Fix issue 22077 - std.sumtype support for copy constructors is inco…

[pbackus]

…mplete

Previously, SumType would define the wrong set of constructors for types
whose copyability varies depending on their mutability--a situation that
was impossible with postblits, but is now possible with copy
constructors.

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Based on pbackus/sumtype@a4cfa0d and pbackus/sumtype@708630f.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22077	normal	`std.sumtype` support for copy constructors is incomplete

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8192"

[pbackus]

Rebased to pick up #8191