Fix Issue 21074 - const lost in mixin (extra case)

[BorisCarvajal]

I didn't notice this case yesterday (#11458).

[dlang-bot]

Thanks for your pull request and interest in making D better, @BorisCarvajal! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21074	regression	const lost in mixin

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 "stable + dmd#11462"