libdw: Handle split DWARF in dwarf_decl_file

libdw: Handle split DWARF in dwarf_decl_file

Calling dwarf_decl_file on a split DWARF DIE fails this assertion:

  dwarf_decl_file.c:72: dwarf_decl_file: Assertion `cu->files != NULL && cu->files != (void *) -1l' failed.

This is because dwarf_decl_file calls dwarf_getsrclines to populate
cu->files.  For normal units, cu->files is cached by dwarf_getsrclines
when it parses the line number information.  However, for split units,
the line number information is parsed for the skeleton unit, then copied
to the split unit's cu->lines.  Split units have their own file name
table, so cu->files is not copied.

The obvious solution is to use dwarf_getsrcfiles instead of relying on
implicit caching.

Also add a test case for dwarf_decl_file.

	* libdw/dwarf_decl_file.c (dwarf_decl_file): Use
	dwarf_getsrcfiles instead of dwarf_getsrclines.
	* tests/Makefile.am (check_PROGRAMS): Add declfiles.
	(TESTS): Add run-declfiles.sh.
	(EXTRA_DIST): Add run-declfiles.sh.
	(declfiles_LDADD): New variable.
	* tests/declfiles.c: New test.
	* tests/run-declfiles.sh: New test.

Signed-off-by: Omar Sandoval <osandov@fb.com>