Drop AbsOverride flag in bodyRetainer methods

This was not noticed before since bodyRetainers were not entered in scope,
so were not subject to override checking.

Author: odersky

compiler/src/dotty/tools/dotc/inlines/Inlines.scala

@@ -229,7 +229,7 @@ object Inlines:
 
     val retainer = meth.copy(
       name = BodyRetainerName(meth.name),
-      flags = meth.flags &~ (Inline | Macro | Override) | Private,
+      flags = (meth.flags &~ (Inline | Macro | Override | AbsOverride)) | Private,
       coord = mdef.rhs.span.startPos).asTerm.entered
     retainer.deriveTargetNameAnnotation(meth, name => BodyRetainerName(name.asTermName))
     DefDef(retainer, prefss =>