make -freg-struct-return visibly a negative alias of -fpcc-struct-return

make -freg-struct-return visibly a negative alias of -fpcc-struct-return

The fact that both options accept negative forms suggests that maybe
they aren't negative forms of each other.  They are, but that isn't
clear even by examining common.opt.  Use NegativeAlias to make it
abundantly clear.

The 'Optimization' keyword next to freg-struct-return was the only
thing that caused flag_pcc_struct_return to be a per-function flag,
and ipa-inline relied on that.  After making it an alias, the
Optimization keyword was no longer operational.  I'm not sure it was
sensible or desirable for flag_pcc_struct_return to be a per-function
setting, but this patch does not intend to change behavior.

for  gcc/ChangeLog

	* common.opt (freg-struct-return): Make it explicitly
	fpcc-struct-return's NegativeAlias.  Copy Optimization...
	(freg-struct-return): ... here.