Since -gsplit-dwarf -g3 will output filename as indirect string, call
set_indirect_string on DW_MACINFO_start_file for -gsplit-dwarf -g3.

gcc/

	PR debug/79342
	* dwarf2out.c (save_macinfo_strings): Call set_indirect_string
	on DW_MACINFO_start_file for -gsplit-dwarf -g3.

gcc/testsuite/

	PR debug/79342
	* gcc.dg/pr79342.: New test.

From-SVN: r263831