Opaque flag is immutable wrt completion only after load

Part of the mechanics of how opaque types work is by
setting the `opaque` flag at the objects that own them.

Author: anatoliykmetyuk

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

@@ -438,7 +438,7 @@ object Flags {
   val FromStartFlags: FlagSet = commonFlags(
     Module, Package, Method, Case,
     HigherKinded, Param, ParamAccessor,
-    Scala2ExistentialCommon, Mutable, Opaque, Touched, JavaStatic,
+    Scala2ExistentialCommon, Mutable, Touched, JavaStatic,
     OuterOrCovariant, LabelOrContravariant, CaseAccessor,
     Extension, NonMember, Implicit, Given, Permanent, Synthetic,
     SuperAccessorOrScala2x, Inline)
@@ -449,7 +449,7 @@ object Flags {
    *  is completed)
    */
   val AfterLoadFlags: FlagSet = commonFlags(
-    FromStartFlags, AccessFlags, Final, AccessorOrSealed, LazyOrTrait, SelfName, JavaDefined)
+    FromStartFlags, Opaque, AccessFlags, Final, AccessorOrSealed, LazyOrTrait, SelfName, JavaDefined)
 
   /** Flags that are not mutated during completion when a certain
    *  condition is satisfied