Fix bug when restoring denotations in Recheck.

Need to copy the denotation, since denotations come with next pointers
which would get scrambled otherwise. The bug was observed when compiling stdlib
under new capture checking implementation.

# Conflicts:
#	compiler/src/dotty/tools/dotc/transform/Recheck.scala

Author: odersky

compiler/src/dotty/tools/dotc/transform/Recheck.scala

@@ -153,7 +153,8 @@ abstract class Recheck extends Phase, SymTransformer:
   /** Change any `ResetPrivate` flags back to `Private` */
   def transformSym(symd: SymDenotation)(using Context): SymDenotation =
     val sym = symd.symbol
-    if sym.isUpdatedAfter(preRecheckPhase) then atPhase(preRecheckPhase)(sym.denot)
+    if sym.isUpdatedAfter(preRecheckPhase)
+    then atPhase(preRecheckPhase)(sym.denot.copySymDenotation())
     else symd
 
   def run(using Context): Unit =