Degrade "case scrutinee not known to diverge for sure" Lint error to warning
Degrade "case scrutinee not known to diverge for sure" Lint error to warning

as proposed in #12435, as it easily gets in the way of development when
it reports false positives.

snoyberg/ghc,olsner/ghc,shlevy/ghc,shlevy/ghc,mettekou/ghc,shlevy/ghc,mettekou/ghc,olsner/ghc,sdiehl/ghc,ezyang/ghc,snoyberg/ghc,shlevy/ghc,snoyberg/ghc,snoyberg/ghc,sdiehl/ghc,olsner/ghc,snoyberg/ghc,shlevy/ghc,mettekou/ghc,sdiehl/ghc,mettekou/ghc,snoyberg/ghc,olsner/ghc,mettekou/ghc,mettekou/ghc,sdiehl/ghc,snoyberg/ghc,olsner/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,olsner/ghc,sdiehl/ghc,ezyang/ghc,olsner/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,mettekou/ghc,shlevy/ghc