Fix missing FreezeList() call to DefaultUnresolvedTypeParameter.Freeze().

This is the probable cause of the following crash (SD UDC-4936):
  System.Collections.Generic.KeyNotFoundException
  System.Collections.Generic.Dictionary`2.get_Item(TKey key)
  ICSharpCode.NRefactory.Utils.FastSerializer.SerializationContext.WriteObjectID(Object instance)
  Write_DefaultUnresolvedTypeParameter(SerializationContext, Object)