libdw and libdwfl srcfiles and srclines fixes for partial_units.

libdw and libdwfl srcfiles and srclines fixes for partial_units.

dwfl_getsrclines would always fail, even when lines were found.
dwarf_decl_file, and other functions relying on srcfiles or srclines would
fail for DIEs in partial_units because stmt_lists on partial_units
were ignored.

Note that dwz contained a bug which makes things fail in __libdw_formptr
for DW_AT_stmt_list with a bogus DW_FORM even with these fixes.
https://bugzilla.redhat.com/show_bug.cgi?id=919755

Signed-off-by: Mark Wielaard <mjw@redhat.com>