Change default to discard temporary local symbols in merge sections.

Change default to discard temporary local symbols in merge sections.

Add --discard-none to keep all local symbols.

gold/
	PR gold/17498
	* object.cc (Sized_relobj_file::do_count_local_symbols): Discard
	temporary locals in merge sections.
	* options.cc (General_options::parse_discard_all): New method.
	(General_options::parse_discard_locals): New method.
	(General_options::parse_discard_none): New method.
	(General_options::General_options): Initialize discard_locals_.
	* options.h (--discard-all): Convert to special option.
	(--discard-locals): Likewise.
	(--discard-none): New option.
	(General_options::discard_all): New method.
	(General_options::discard_locals): New method.
	(General_options::discard_sec_merge): New method.
	(General_options::Discard_locals): New enum.
	(General_options::discard_locals_): New data member.