GDB and LLDB pretty printers have some common functionality
and also access some common information, such as the layout of
standard library types. So far, this information has been
duplicated in the two pretty printing python modules. This
commit introduces a common module used by both debuggers.
LLDB doesn't allow for reading 'artifical' fields (fields that are generated by the compiler). So do not mark, slice fields, enum discriminants, and GcBox value fields as artificial.
