Support using switch without importing SwitchContext (bp #1595)

[mergify]

This is an automated backport of pull request #1595 done by Mergify

Cherry-pick of 9c4f14f has failed:

On branch mergify/bp/3.3.x/pr-1595
Your branch is up to date with 'origin/3.3.x'.

You are currently cherry-picking commit 9c4f14fb.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:

	modified:   src/main/scala/chisel3/util/Conditional.scala
	modified:   src/test/scala/chiselTests/SwitchSpec.scala

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)

	deleted by us:   docs/src/wiki-deprecated/cookbook.md

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