Warn on non-exhaustive for-comprehension patterns

We assume patterns are exhaustive when desugaring with
`-Zirrefutable-generator-patterns` but we should still emit a warning if
we think they might not be.

Author: puffnfresh

src/reflect/scala/reflect/internal/TreeGen.scala

@@ -622,7 +622,7 @@ abstract class TreeGen {
             body) setPos splitpos
         case None =>
           atPos(splitpos) {
-            mkVisitor(List(CaseDef(pat, EmptyTree, body)), checkExhaustive = false)
+            mkVisitor(List(CaseDef(pat, EmptyTree, body)), checkExhaustive = settings.ZirrefutableGeneratorPatterns)
           }
       }
     }