SymDenotations: do not use deprecated BitSet#toImmutable

As explained in the deprecated message:
"method toImmutable in class BitSet is deprecated: If this BitSet
contains a value that is 128 or greater, the result of this method is an
'immutable' BitSet that shares state with this mutable BitSet. Thus, if
the mutable BitSet is modified, it will violate the immutability of the
result"

Author: smarter

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

@@ -7,7 +7,7 @@ import Types._, Flags._, Decorators._, DenotTransformers._, StdNames._, Scopes._
 import NameOps._, NameKinds._
 import Scopes.Scope
 import collection.mutable
-import collection.immutable.BitSet
+import collection.BitSet
 import scala.reflect.io.AbstractFile
 import Decorators.SymbolIteratorDecorator
 import ast._
@@ -1412,7 +1412,7 @@ object SymDenotations {
           to
       }
       val bcs = classSymbol :: addParentBaseClasses(classParents, Nil)
-      val scbits = seen.toImmutable
+      val scbits = seen
       if (isFullyCompleted) {
         myBaseClasses = bcs
         mySuperClassBits = scbits