Support literals and DontCare in DataView targets (backport #3389)

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This is an automatic backport of pull request #3389 done by Mergify.
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	modified:   core/src/main/scala/chisel3/experimental/dataview/package.scala

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Original PR Body

The best example of why this is useful is the test showing some possible uses with Valid:

object ValidExtensions {
  implicit def view[T <: Data] = DataView[T, Valid[T]](
    x => Valid(x.cloneType), // Valid will strip direction with `Output(...)` anyway
    _ -> _.bits,
    (_, v) => true.B -> v.valid
  )
}

This enables you to box any T as a Valid[T] where the valid bit is the literal true:

import ValidExtensions._
class MyModule extends Module {
  val in = IO(Input(UInt(8.W)))
  val out= IO(Output(Valid(UInt(8.W))))
  out := in.viewAs[Valid[UInt]]
}

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

• Feature (or new API)

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: 3.5.x or 3.6.x depending on impact, API modification or big change: 5.0.0)?
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• Did you do one of the following when ready to merge:
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