Avoid returning stale symbols from NamedType#symbol

There's a scenario where a NamedType is created with an initial symbol
but no denotation yet. If the denotation of that NamedType is first
computed in a subsequent run, the initially given symbol might be stale,
so the symbol should be recomputed from the denotation.

Author: odersky

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

@@ -1737,7 +1737,9 @@ object Types {
     def symbol(implicit ctx: Context): Symbol = {
       val now = ctx.period
       if (checkedPeriod == now ||
-          lastDenotation == null && lastSymbol != null) lastSymbol
+          lastDenotation == null && lastSymbol != null &&
+          (checkedPeriod.runId == now.runId || ctx.stillValid(lastSymbol)))
+        lastSymbol
       else denot.symbol
     }