Guard against completions in wrong runs

We get sometimes stake symbol error because a completer
is run in its creation context, which comes from a previous run.
This should not happen. Adding an assert to flag the erronenous
completion when it happens.

Author: odersky

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

@@ -756,6 +756,7 @@ class Namer { typer: Typer =>
           else levels(c.outer) + 1
         completions.println(s"!!!completing ${denot.symbol.showLocated} in buried typerState, gap = ${levels(ctx)}")
       }
+      assert(ctx.runId == creationContext.runId, "completing $denot in wrong run ${ctx.runId}, was created in ${creationContext.runId}")
       completeInCreationContext(denot)
     }