libdw: Use correct CU to resolve file names in dwarf_decl_file.

libdw: Use correct CU to resolve file names in dwarf_decl_file.

dwarf_decl_file uses dwarf_attr_integrate to get the DW_AT_decl_file
attribute. This means the attribute might come from a different DIE
in a different CU. If so, we need to use the CU associated with the
attribute, not the original DIE, to resolve the file name.

Also add a bit more documentation to dwarf_attr_integrate explaining
that the attribute returned might come from a different CU (and even
different Dwarf).

Signed-off-by: Mark Wielaard <mark@klomp.org>