Don't warn when 0.U used as value for 0-bit BundleLit field (backport #4097)

[mergify]

Having to add this special-case is a shame, but worthwhile. Basically, if a user writes something like:

(new MyBundle).Lit(_.field1 -> whatever, _.field2 -> (some * complex / expression).U)

We should not warn if field2 is 0-bit and that expression evaluates to 0, it's way too onerous to expect the user to handle passing 0.W to .U in this case. I think a similar thing will come up when we start warning/removing implicit truncation. It's a special-case, but it's pragmatic.

The best fix would be for 0.U to start being zero-width, but that has a ton of knock on effects and should be considered a breaking change and handled as such.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
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• Did you add appropriate documentation in docs/src?
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• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• Bugfix

Desired Merge Strategy

• Squash

Release Notes

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
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• Did you do one of the following when ready to merge:
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This is an automatic backport of pull request #4097 done by [Mergify](https://mergify.com).