Fix Issue 15940 - ImplicitConversionTargets and class alias in struct

[ghost]

Based on PR #4342. I don't really completely understand the implications of this change, but all tests pass and as the original PR has been more or less approved by JackStouffer and I think, that PR was only closed, because the author of the PR left, I resubmit it.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	15940	normal	ImplicitConversionTargets and class alias in struct

Testing this PR locally

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

dub fetch digger
dub run digger -- build "master + phobos#7313"

[FeepingCreature]

As a side effect, this introduced a "bug" - class A; ImplicitConversionTargets!(const A) is now wrongly (Object) instead of, as previous, wrongly (). Correct would be (const Object).