fix Issue 19788 - compiler crash on slicing a enum typed as vector

[WalterBright]

Actually, just add a test case for it, as it seems to work.

[dlang-bot]

Thanks for your pull request, @WalterBright!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19788	normal	compiler crash on slicing a enum typed as vector

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• Regression or critical bug fixes should always target the stable branch. Learn more about rebasing to stable or the D release process.

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Testing this PR locally

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dub run digger -- build "master + dmd#12043"