Fix Bugzilla Issue 24760 - ICE on variadic after default argument

[RazvanN7]

This used to be illegal prior to the addition of named arguments, however, now it has become legal. One way to fix this would have been to disallow this pattern, but since the fix is quite simple I just went ahead and accepted it as valid code.

Not targeting stable as I'm having doubts that this is a pattern we should support.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	24760	normal	ICE on variadic after default argument

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#16834"

[thewilsonator]

ICE should go to stable. Otherwise looks good

[RazvanN7]

@thewilsonator ok. done.