Fix issue 22910 - return scope struct member functions allow returning this by ref

[dkorpel]

Following up #13693
Blocked by: dlang/druntime#3782 dlang/phobos#8416
The supplemental error is not right, but that's a separate issue.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22910	normal	[dip1000] return scope struct member functions allow returning `this` by ref

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 + dmd#13860"

[dkorpel]

It's unblocked now

[RazvanN7]

Shouldn't we issue a deprecation for this? Or do we consider that dip1000 is experimental and therefore breaking changes are allowed?

[dkorpel]

Or do we consider that dip1000 is experimental and therefore breaking changes are allowed?

Code behind a -preview switch may break is the policy: #13691 (comment)

This PR is no different than any of the previous 'accepts invalid' fixes of dip1000 in that regard.