Fix 21791 - Stack overflow for forward-referenced enum initializer

[MoonlightSentinel]

Ensure that semantic for the EnumMember chosen as .init was already done.

The previous check was never hit because sematicRun is changed before analyzing the enum body.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21791	normal	Stack overflow for forward-referenced enum initializer

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#12346"