Make rewriteCtFlavor lazy in the coercion for Derived evidence
Make rewriteCtFlavor lazy in the coercion for Derived evidence

I think I accidentally introduced this bug a month ago when
refactoring. It's a bit non-obvious, but since Derived constraints
have no evidence, we mustn't be strict in it.  Now there's a big
comment to prevent this bug happening again.

This fixes Trac #7384.

vTurbine/ghc,jstolarek/ghc,vTurbine/ghc,mettekou/ghc,gridaphobe/ghc,GaloisInc/halvm-ghc,spacekitteh/smcghc,lukexi/ghc,green-haskell/ghc,snoyberg/ghc,mfine/ghc,holzensp/ghc,elieux/ghc,tjakway/ghcjvm,jstolarek/ghc,acowley/ghc,nathyong/microghc-ghc,ezyang/ghc,hferreiro/replay,sdiehl/ghc,tibbe/ghc,forked-upstream-packages-for-ghcjs/ghc,vikraman/ghc,nushio3/ghc,acowley/ghc,elieux/ghc,vikraman/ghc,gcampax/ghc,vTurbine/ghc,AlexanderPankiv/ghc,GaloisInc/halvm-ghc,vikraman/ghc,snoyberg/ghc,bitemyapp/ghc,AlexanderPankiv/ghc,fmthoma/ghc,olsner/ghc,mcschroeder/ghc,da-x/ghc,christiaanb/ghc,ezyang/ghc,ezyang/ghc,ekmett/ghc,ghc-android/ghc,gcampax/ghc,TomMD/ghc,lukexi/ghc,shlevy/ghc,spacekitteh/smcghc,ryantm/ghc,frantisekfarka/ghc-dsi,lukexi/ghc,nkaretnikov/ghc,nathyong/microghc-ghc,GaloisInc/halvm-ghc,holzensp/ghc,olsner/ghc,anton-dessiatov/ghc,snoyberg/ghc,forked-upstream-packages-for-ghcjs/ghc,oldmanmike/ghc,siddhanathan/ghc,lukexi/ghc-7.8-arm64,anton-dessiatov/ghc,forked-upstream-packages-for-ghcjs/ghc,green-haskell/ghc,da-x/ghc,frantisekfarka/ghc-dsi,nathyong/microghc-ghc,ekmett/ghc,tibbe/ghc,urbanslug/ghc,da-x/ghc,ghc-android/ghc,forked-upstream-packages-for-ghcjs/ghc,holzensp/ghc,mettekou/ghc,spacekitteh/smcghc,da-x/ghc,fmthoma/ghc,ryantm/ghc,urbanslug/ghc,siddhanathan/ghc,ml9951/ghc,shlevy/ghc,fmthoma/ghc,ml9951/ghc,sgillespie/ghc,AlexanderPankiv/ghc,gridaphobe/ghc,snoyberg/ghc,gridaphobe/ghc,hferreiro/replay,ml9951/ghc,AlexanderPankiv/ghc,vTurbine/ghc,tjakway/ghcjvm,sdiehl/ghc,acowley/ghc,olsner/ghc,christiaanb/ghc,nushio3/ghc,da-x/ghc,mcschroeder/ghc,oldmanmike/ghc,mettekou/ghc,tjakway/ghcjvm,siddhanathan/ghc,bitemyapp/ghc,TomMD/ghc,bitemyapp/ghc,nushio3/ghc,mfine/ghc,elieux/ghc,hferreiro/replay,ml9951/ghc,oldmanmike/ghc,gcampax/ghc,tjakway/ghcjvm,anton-dessiatov/ghc,sdiehl/ghc,sgillespie/ghc,sgillespie/ghc,nkaretnikov/ghc,nathyong/microghc-ghc,frantisekfarka/ghc-dsi,nushio3/ghc,lukexi/ghc-7.8-arm64,ghc-android/ghc,jstolarek/ghc,shlevy/ghc,GaloisInc/halvm-ghc,gridaphobe/ghc,sdiehl/ghc,frantisekfarka/ghc-dsi,GaloisInc/halvm-ghc,oldmanmike/ghc,GaloisInc/halvm-ghc,nkaretnikov/ghc,oldmanmike/ghc,acowley/ghc,mfine/ghc,nkaretnikov/ghc,urbanslug/ghc,mfine/ghc,forked-upstream-packages-for-ghcjs/ghc,gridaphobe/ghc,ghc-android/ghc,ekmett/ghc,christiaanb/ghc,ghc-android/ghc,siddhanathan/ghc,nkaretnikov/ghc,anton-dessiatov/ghc,green-haskell/ghc,GaloisInc/halvm-ghc,nkaretnikov/ghc,fmthoma/ghc,wxwxwwxxx/ghc,gridaphobe/ghc,wxwxwwxxx/ghc,TomMD/ghc,tibbe/ghc,snoyberg/ghc,gcampax/ghc,mettekou/ghc,hferreiro/replay,siddhanathan/ghc,christiaanb/ghc,fmthoma/ghc,snoyberg/ghc,siddhanathan/ghc,vikraman/ghc,fmthoma/ghc,lukexi/ghc-7.8-arm64,sdiehl/ghc,wxwxwwxxx/ghc,mettekou/ghc,mcschroeder/ghc,lukexi/ghc-7.8-arm64,TomMD/ghc,nushio3/ghc,sgillespie/ghc,urbanslug/ghc,sgillespie/ghc,sgillespie/ghc,anton-dessiatov/ghc,olsner/ghc,oldmanmike/ghc,shlevy/ghc,shlevy/ghc,ryantm/ghc,gcampax/ghc,bitemyapp/ghc,ryantm/ghc,ezyang/ghc,christiaanb/ghc,tibbe/ghc,tjakway/ghcjvm,bitemyapp/ghc,vTurbine/ghc,wxwxwwxxx/ghc,vikraman/ghc,spacekitteh/smcghc,anton-dessiatov/ghc,elieux/ghc,elieux/ghc,hferreiro/replay,da-x/ghc,holzensp/ghc,TomMD/ghc,ezyang/ghc,TomMD/ghc,shlevy/ghc,wxwxwwxxx/ghc,acowley/ghc,green-haskell/ghc,ghc-android/ghc,forked-upstream-packages-for-ghcjs/ghc,AlexanderPankiv/ghc,TomMD/ghc,acowley/ghc,olsner/ghc,tjakway/ghcjvm,ml9951/ghc,ghc-android/ghc,sdiehl/ghc,fmthoma/ghc,lukexi/ghc,ml9951/ghc,spacekitteh/smcghc,nathyong/microghc-ghc,shlevy/ghc,gcampax/ghc,acowley/ghc,AlexanderPankiv/ghc,nushio3/ghc,hferreiro/replay,sdiehl/ghc,ryantm/ghc,jstolarek/ghc,tjakway/ghcjvm,siddhanathan/ghc,wxwxwwxxx/ghc,gridaphobe/ghc,snoyberg/ghc,ezyang/ghc,mfine/ghc,hferreiro/replay,frantisekfarka/ghc-dsi,oldmanmike/ghc,vTurbine/ghc,jstolarek/ghc,mfine/ghc,vikraman/ghc,gcampax/ghc,tibbe/ghc,elieux/ghc,ml9951/ghc,vTurbine/ghc,anton-dessiatov/ghc,urbanslug/ghc,elieux/ghc,AlexanderPankiv/ghc,christiaanb/ghc,lukexi/ghc,mfine/ghc,olsner/ghc,christiaanb/ghc,forked-upstream-packages-for-ghcjs/ghc,nathyong/microghc-ghc,mettekou/ghc,ezyang/ghc,lukexi/ghc-7.8-arm64,urbanslug/ghc,ekmett/ghc,ekmett/ghc,green-haskell/ghc,urbanslug/ghc,nkaretnikov/ghc,mettekou/ghc,mcschroeder/ghc,nushio3/ghc,vikraman/ghc,ml9951/ghc,mcschroeder/ghc,mcschroeder/ghc,holzensp/ghc,wxwxwwxxx/ghc,mcschroeder/ghc,da-x/ghc,sgillespie/ghc,nathyong/microghc-ghc,olsner/ghc