Support separately elaborating definition and instance in ChiselStage (backport #2512)

[mergify]

This is an automatic backport of pull request #2512 done by Mergify.
Cherry-pick of a0aa4d1 has failed:

On branch mergify/bp/3.5.x/pr-2512
Your branch is up to date with 'origin/3.5.x'.

You are currently cherry-picking commit a0aa4d15.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   src/test/scala/chiselTests/experimental/hierarchy/SeparateElaborationSpec.scala

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   core/src/main/scala/chisel3/experimental/hierarchy/Definition.scala
	both modified:   core/src/main/scala/chisel3/experimental/hierarchy/Instance.scala
	both modified:   core/src/main/scala/chisel3/internal/Builder.scala

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.com