coffgrok access of u.auxent.x_sym.x_tagndx.p

coffgrok access of u.auxent.x_sym.x_tagndx.p

u.auxent.x_sym.x_tagndx is a union.  The p field is only valid when
fix_tag is set.  This patch fixes code in coffgrok.c that accessed the
field without first checking fix_tag, and removes a whole lot of code
validating bogus pointers to prevent segfaults (which no longer
happen, I checked the referenced PR 17512 testcases).  The patch also
documents this in the fix_tag comment, makes is_sym a bitfield, and
sorts the selecter fields a little.

bfd/
	* coffcode.h (combined_entry_type): Make is_sym a bitfield.
	Sort and comment on union selectors.
	* libcoff.h: Regenerate.
binutils/
	* coffgrok.c (do_type): Make aux a combined_entry_type.  Test
	fix_tag before accessing u.auxent.x_sym.x_tagndx.p.  Remove
	now unnecessary pointer bounds checking.