Check that we pickle a definition before its references

... except for pattern-bound symbols because in `case x: List[t]` we
pickle `List[t]` before we pickle `t` itself.

The issue with t1957.scala is fixed (after staying as a TODO for 5
years!) by the level-checking improvements in the previous commits of
this PR.

compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala

@@ -89,11 +89,6 @@ class TreePickler(pickler: TastyPickler) {
     case Some(label) =>
       if (label != NoAddr) writeRef(label) else pickleForwardSymRef(sym)
     case None =>
-      // See pos/t1957.scala for an example where this can happen.
-      // I believe it's a bug in typer: the type of an implicit argument refers
-      // to a closure parameter outside the closure itself. TODO: track this down, so that we
-      // can eliminate this case.
-      report.log(i"pickling reference to as yet undefined $sym in ${sym.owner}", sym.srcPos)
       pickleForwardSymRef(sym)
   }
 
@@ -206,6 +201,8 @@ class TreePickler(pickler: TastyPickler) {
       }
       else if (tpe.prefix == NoPrefix) {
         writeByte(if (tpe.isType) TYPEREFdirect else TERMREFdirect)
+        if !symRefs.contains(sym) && !sym.isPatternBound && !sym.hasAnnotation(defn.QuotedRuntimePatterns_patternTypeAnnot) then
+          report.error(i"pickling reference to as yet undefined $tpe with symbol ${sym}", sym.srcPos)
         pickleSymRef(sym)
       }
       else tpe.designator match {