Address #11245: Ensure the non-matched list is always non-empty
Address #11245: Ensure the non-matched list is always non-empty

When there is an uncovered vector of length 0 (which in turn means that
it represents a guard failure) print "(incomplete guards)" instead of an
empty list of non-covered vectors.

Test Plan: validate

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1717

GHC Trac Issues: #11245

nushio3/ghc,oldmanmike/ghc,ezyang/ghc,snoyberg/ghc,mettekou/ghc,vikraman/ghc,ezyang/ghc,gridaphobe/ghc,nushio3/ghc,snoyberg/ghc,olsner/ghc,vTurbine/ghc,tjakway/ghcjvm,nushio3/ghc,gridaphobe/ghc,tjakway/ghcjvm,olsner/ghc,sdiehl/ghc,vikraman/ghc,gridaphobe/ghc,vTurbine/ghc,gridaphobe/ghc,ezyang/ghc,vikraman/ghc,sdiehl/ghc,shlevy/ghc,mcschroeder/ghc,vTurbine/ghc,sdiehl/ghc,oldmanmike/ghc,nushio3/ghc,tjakway/ghcjvm,shlevy/ghc,sdiehl/ghc,mcschroeder/ghc,olsner/ghc,vTurbine/ghc,sdiehl/ghc,nushio3/ghc,mettekou/ghc,oldmanmike/ghc,oldmanmike/ghc,ezyang/ghc,ezyang/ghc,gridaphobe/ghc,vTurbine/ghc,nushio3/ghc,mettekou/ghc,mcschroeder/ghc,tjakway/ghcjvm,shlevy/ghc,olsner/ghc,mettekou/ghc,vTurbine/ghc,snoyberg/ghc,tjakway/ghcjvm,tjakway/ghcjvm,mettekou/ghc,sgillespie/ghc,tjakway/ghcjvm,sgillespie/ghc,sgillespie/ghc,vikraman/ghc,sgillespie/ghc,vikraman/ghc,snoyberg/ghc,olsner/ghc,sgillespie/ghc,oldmanmike/ghc,gridaphobe/ghc,olsner/ghc,mcschroeder/ghc,mcschroeder/ghc,sdiehl/ghc,vikraman/ghc,nushio3/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,mettekou/ghc,mcschroeder/ghc,olsner/ghc,snoyberg/ghc,shlevy/ghc,sgillespie/ghc,shlevy/ghc,sgillespie/ghc,vTurbine/ghc,mcschroeder/ghc,mettekou/ghc,snoyberg/ghc,snoyberg/ghc,vikraman/ghc,oldmanmike/ghc,shlevy/ghc,gridaphobe/ghc,ezyang/ghc,oldmanmike/ghc