Make 'private function is not accessible' deprecation an error.

[FeepingCreature]

Affects issue 21275, 23947.
Followup to #15282

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	21275	enhancement	Overload resolution bypasses private access
✗	23947	major	If a class overloads a method mixing private and public and the last overload is public, the method is always public.

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#15319"

[RazvanN7]

Could you please also add a changelog entry?

[FeepingCreature]

Changelog added.

[FeepingCreature]

Ah, hm.

Note that the changelog as written is a bit iffy: the deprecation period for 21275 started with 2.094, but my 23947 was merged a few days ago and absorbed 21275's deprecation message because the check for 23947 also covered 21275.

I don't think 23947 deserves a deprecation of its own. Then again, I'm not sure why 21275 wasn't fixed immediately to begin with, particularly considering the trivial remedy of "just make whatever you couldn't access, public instead".

If 23947 got its own deprecation period, I'd have to split them back into separate codepaths.

[RazvanN7]

All of these are aspects of the same bug so it makes sense to just turn the deprecation into an error for all.