Throw an exception for NoSymbol.owner

This preserves the former behavior on NoDenotation.

Author: odersky

compiler/src/dotty/tools/dotc/core/Symbols.scala

@@ -891,6 +891,7 @@ object Symbols {
   }
 
   @sharable val NoSymbol: Symbol = new Symbol(NoCoord, 0) {
+    override def owner(implicit ctx: Context): Symbol = throw new AssertionError("NoSymbol.owner")
     override def associatedFile(implicit ctx: Context): AbstractFile = NoSource.file
     override def recomputeDenot(lastd: SymDenotation)(implicit ctx: Context): SymDenotation = NoDenotation
   }