Fix 14954 - extern opaque struct instance doesn't compile

[MoonlightSentinel]

Don't raise an error if the VarDeclaration is marked as extern.

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Based on #12239 to avoid the ugly merge conflict.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	14954	normal	extern opaque struct instance doesn't compile
✓	21668	minor	Cannot declare ref parameter of opaque type

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#12240"