fix #22383 - bitfield .init value contains void elements

[ibuclaw]

defaultInit wasn't adapted to deal with bitfields.

Closes #22383.

[dlang-bot]

Thanks for your pull request, @ibuclaw!

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Testing this PR locally

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dub run digger -- build "master + dmd#22395"

[ibuclaw]

@thewilsonator maybe check if the issue ref is correct.