[Scala3] Redux: Convert cloneType to an extension method

[adkian-sifive]

Redux of #3771. Turns out we need this for Scala3 after all

Ran into errors related to this while adding Scala3 cross-compilation support. Here's a sample of what an error with Dotty looks like:

[error] -- [E007] Type Mismatch Error: chisel3/core/src/main/scala/chisel3/AggregateImpl.scala:838:4 
[error] 838 |    clone
[error]     |    ^^^^^
[error]     |    Found:    (clone : chisel3.Record & chisel3.RecordImpl)
[error]     |    Required: (RecordImpl.this : chisel3.Record & chisel3.RecordImpl)

I suspect this is due to the tightening of the type system in Scala3, specifically changes which make this.type a more stricter representation of the current instance. I think this SIP might have a more detailed specification of the related changes: https://docs.scala-lang.org/sips/42.type.html#related-scala-issues-resolved-by-the-literal-types-implementation

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?
• [ ] Did you specify the type of improvement?
• [ ] Did you add appropriate documentation in docs/src?
• [ ] Did you request a desired merge strategy?
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Type of Improvement

• API Modification
• Internal or build-related (includes code refactoring/cleanup)

Desired Merge Strategy

• Squash: The PR will be squashed and merged (choose this if you have no preference).

Release Notes

Move cloneType to an extension method in preparation for Scala3

Reviewer Checklist (only modified by reviewer)

• [ ] Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• [ ] Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
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• [ ] Did you do one of the following when ready to merge:
  • [ ] Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
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