Add bool_constant_dom, use it for DW_FORM_flag{,_present}
Support @decl_file

- Attribute slots need to carry around a DIE offset, so that it's
  possible to get from the attribute to its DIE.  @decl_file uses this
  to get to CU DIE, which libdw needs to extract file information.
Fix parsing of !{}
Add a layout comment to union valfile_slot