Fix Issue 13063 - enum is allowed as storage class for functions

[ntrel]

No description provided.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	13063	normal	`enum` is allowed as storage class for functions

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#15405"

[RazvanN7]

Maybe also add a changelog entry? I expect that there might be code out there abusing this pattern.

[CyberShadow]

@ntrel @RazvanN7 @WalterBright This should have been a deprecation, it caused downstream breakage with no warning:
https://aur.archlinux.org/packages/btdu#comment-928456