Make isView safe outside of Builder context (backport #4228)

[mergify]

Contemplating views in naming (#4222) exposed a longstanding bug that I patched out recently in #4147. Basically isView does not work outside of Builder contexts. Now it is safe.

We do have lots of tests for things like literals outside of Builder contexts, but they didn't trigger this because ViewParent is a lazy val and thus if any Chisel elaboration has occurred on the same thread, then the problem doesn't manifest. Thus this "test" looks very strange, but by virtue of defining a literal assigned to a val first thing in a ScalaTest spec, it ensures no Chisel elaboration has run before when this line executes, and the naming plugin + #4222 ensures that this value is checked if it's a view.

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?
• 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")
• 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:
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This is an automatic backport of pull request #4228 done by [Mergify](https://mergify.com).