Adds special treatment to Predef.classOf (#21523)

This is an updated PR of #20945, making the global initialization
checker skips the analysis of `Predef.classOf` in scala2-library, since
it is a stub method in tasty and is replaced by actual class
representations in the backend. This prevents related warnings when
running test_scala2_library_tasty.

---------

Co-authored-by: EnzeXing <enzexing12@gmail.com>

Author: EnzeXing

compiler/src/dotty/tools/dotc/transform/init/Objects.scala

@@ -702,6 +702,9 @@ class Objects(using Context @constructorOnly):
           val arr = OfArray(State.currentObject, summon[Regions.Data])
           Heap.writeJoin(arr.addr, args.map(_.value).join)
           arr
+        else if target.equals(defn.Predef_classOf) then
+          // Predef.classOf is a stub method in tasty and is replaced in backend
+          Bottom
         else if target.hasSource then
           val cls = target.owner.enclosingClass.asClass
           val ddef = target.defTree.asInstanceOf[DefDef]