Deprecate calling .viewAs on non-hardware (backport #3395) (#3400)

* Deprecate calling .viewAs on non-hardware (#3395)

It's an error in most cases already, and in the cases it isn't, make it
a warning. Also fix a crash that could occur when viewing a non-Data
containing non-hardware Data.

(cherry picked from commit 1e524de9

)

# Conflicts:
#	src/test/scala/chiselTests/experimental/DataView.scala

* Resolve backport conflicts

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Co-authored-by: Jack Koenig <koenig@sifive.com>