COFF: Check for symbols defined in discarded section

COFF: Check for symbols defined in discarded section

For LTO, a symbol may defined in discarded section.  We should mark it
as undefined so that LTO plugin will make IR definition available.

	PR ld/24267
	* coffgen.c (_bfd_coff_section_already_linked): Skip discarded
	section.
	* cofflink.c (coff_link_add_symbols): Check for symbols defined
	in discarded section.