Try to avoid datarace involving NoDenotation

The thread-shared value NoDenotation has a validity with  run-id that got updated to the run-id
of the current thread. This caused a partest failure with a "demanding denotation outside its range"
error. We now treat NoDenotation specially in current, hoping this will avoid the condition.

Author: odersky

src/dotty/tools/dotc/core/Denotations.scala

@@ -556,7 +556,7 @@ object Denotations {
      */
     private def bringForward()(implicit ctx: Context): SingleDenotation = this match {
       case denot: SymDenotation if ctx.stillValid(denot) =>
-        if (denot.exists) assert(ctx.runId > validFor.runId, s"denotation $denot invalid in run ${ctx.runId}. ValidFor: $validFor")
+        assert(ctx.runId > validFor.runId, s"denotation $denot invalid in run ${ctx.runId}. ValidFor: $validFor")
         var d: SingleDenotation = denot
         do {
           d.validFor = Period(ctx.period.runId, d.validFor.firstPhaseId, d.validFor.lastPhaseId)
@@ -592,7 +592,9 @@ object Denotations {
         assert(false)
       }
 
-      if (valid.runId != currentPeriod.runId) initial.bringForward.current
+      if (valid.runId != currentPeriod.runId)
+        if (exists) initial.bringForward.current
+        else this
       else {
         var cur = this
         if (currentPeriod.code > valid.code) {