Give a warning when constraints are unsatisfiable

We will get some other error when checking bounds in PostTyper,
unless we get a failure in Typer which might produce questionable errors. Having
a warning here might be better than nothing.

```scala
-- Warning: tests/neg/i4721.scala:2:39 -----------------------------------------
2 |  def main(args: Array[String]):Unit = m(1) // error
  |                                       ^
  |       Unsatisfiable type parameter constraints in polymorphic application
-- [E007] Type Mismatch Error: tests/neg/i4721.scala:2:41 ----------------------
2 |  def main(args: Array[String]):Unit = m(1) // error
  |                                         ^
  |                                         found:    Int(1)
  |                                         required: String
  |
one warning found
one error found
```

Author: Blaisorblade

compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala

@@ -480,7 +480,12 @@ object ProtoTypes {
       else tl
     val added = ensureFresh(tl)
     val tvars = if (addTypeVars) newTypeVars(added) else Nil
-    ctx.typeComparer.addToConstraint(added, tvars.tpes.asInstanceOf[List[TypeVar]])
+    // addToConstraint can return false if the constraints have no solution, but it seems easier to detect and report
+    // any resulting bound violations later in PostTyper (see #4946). We still give a warning because the subsequent
+    // errors might be misleading.
+    if (!ctx.typeComparer.addToConstraint(added, tvars.tpes.asInstanceOf[List[TypeVar]]))
+      ctx.warning(s"Unsatisfiable type parameter constraints in polymorphic application", owningTree.pos)
+
     (added, tvars)
   }