Output filepath strings in .debug_line_str for DWARF5

Output filepath strings in .debug_line_str for DWARF5

DWARF5 has a new string table specially for file paths. .debug_line
file and dir tables reference strings in .debug_line_str.  If a
.debug_line_str section is emitted then also place CU DIE file
names and comp dirs there.

gcc/ChangeLog:

	* dwarf2out.c (add_filepath_AT_string): New function.
	(asm_outputs_debug_line_str): Likewise.
	(add_filename_attribute): Likewise.
	(add_comp_dir_attribute): Call add_filepath_AT_string.
	(gen_compile_unit_die): Call add_filename_attribute for name.
	(init_sections_and_labels): Init debug_line_str_section when
	asm_outputs_debug_line_str return true.
	(dwarf2out_early_finish): Remove DW_AT_name and DW_AT_comp_dir
	hack and call add_filename_attribute for the remap_debug_filename.