Don't crash with bad variants, add LabeledRow invariant.
Don't crash with bad variants, add LabeledRow invariant.

Originally I thought LabeledRow would go away after inference, but if
there are functions with implicit arguments it can last until the end
of Simp. Once everything has been simplified, though, LabeledRow atoms
should have been substituted away.

google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang