Strip global symbol defined in discarded section

When a global symbol is defined in COMDAT group, we shouldn't leave an
undefined symbol in symbol table when the symbol section is discarded
unless there is a reference to the symbol outside of COMDAT group.

bfd/

	PR ld/17550
	* elf-bfd.h (elf_link_hash_entry): Update comments for indx,
	documenting that indx == -3 if symbol is defined in a discarded
	section.
	* elflink.c (elf_link_add_object_symbols): Set indx to -3 if
	symbol is defined in a discarded section.
	(elf_link_output_extsym): Strip a global symbol defined in a
	discarded section.

ld/

	PR ld/17550
	* testsuite/ld-elf/pr17550-1.s: New file.
	* testsuite/ld-elf/pr17550-2.s: Likewise.
	* testsuite/ld-elf/pr17550-3.s: Likewise.
	* testsuite/ld-elf/pr17550-4.s: Likewise.
	* testsuite/ld-elf/pr17550a.d: Likewise.
	* testsuite/ld-elf/pr17550b.d: Likewise.
	* testsuite/ld-elf/pr17550c.d: Likewise.
	* testsuite/ld-elf/pr17550d.d: Likewise.