debug: Fix up DWARF 5 -g -flto -ffat-lto-objects [PR98765]

debug: Fix up DWARF 5 -g -flto -ffat-lto-objects [PR98765]

As mentioned in the PR, with -gdwarf-5 (or -g now) -flto -ffat-lto-objects,
users can't strip the LTO sections with
strip -p -R .gnu.lto_* -R .gnu.debuglto_* -N __gnu_lto_v1
anymore when GCC is configured against recent binutils.

The problem is that in that case .gnu.debuglto_.debug_line_str section is
then used, which is fine for references to strings in .gnu.debuglto_.*
sections, but not when those references are in .debug_info section too;
those should really reference separate strings in .debug_line_str section.

For .gnu.debuglto_.debug_str vs. .debug_str we handle it right, we
reset_indirect_string the strings and thus force creation of new labels for
the second time.
But for DW_FORM_line_strp as the patch shows, there were multiple problems.
First one was that reset_indirect_string, even when called through traverse
on debug_line_str_hash, didn't do anything at all (fixed by first hunk).
The second bug was that the DW_FORM_line_strp strings, which were supposed
to be only visible through debug_line_str_hash, leaked into debug_str_hash
(second hunk).
And the third thing is that when we reset debug_line_str_hash, we should
still make those strings DW_FORM_line_strp if they are accessed.
One could do it by reinstantiating DW_FORM_line_strp right away in
reset_indirect_string and not clear debug_line_str_hash, but that has the
disadvantage that we then force emitting .debug_line_str strings that aren't
really needed - we need those from the CU DIEs' DW_AT_name and
DW_AT_comp_dir attributes, but when emitting .debug_line section through
assembler, we don't need to emit the strings we only needed for
.gnu.debuglto_.debug_line which is always emitted by the compiler.

2021-01-20  Jakub Jelinek  <jakub@redhat.com>

	PR debug/98765
	* dwarf2out.c (reset_indirect_string): Also reset indirect strings
	with DW_FORM_line_strp form.
	(prune_unused_types_update_strings): Don't add into debug_str_hash
	indirect strings with DW_FORM_line_strp form.
	(adjust_name_comp_dir): New function.
	(dwarf2out_finish): Call it on CU DIEs after resetting
	debug_line_str_hash.