[Ada] Removal of various ??? comments plus related code changes

[Ada] Removal of various ??? comments plus related code changes

gcc/ada/

	* checks.adb: Remove calls of Set_Do_Tag_Check (N, False).
	* einfo.ads: Remove hanging unneeded ??? comment ("this real description
	was clobbered").
	* exp_util.ads (Insert_Actions_After): Remove ??? from spec comment.
	* gen_il-fields.ads (Opt_Field_Enum): Remove literals
	Do_Accessibility_Check and Do_Tag_Check.
	* gen_il-gen-gen_nodes.adb: Remove all calls to Sm for
	Do_Accessibility_Check and Do_Tag_Check.
	* sem_type.ads (Is_Subtype_Of): Remove obsolete ???
	comment (function is not limited to scalar subtypes).
	* sem_util.ads (Is_Local_Variable_Reference): Revise comment to
	mention out-mode parameters as well, and remove ???.
	(Propagate_Concurrent_Flags): Minor reformatting.
	(Propagate_Invariant_Attributes): Typo fix.
	(Propagate_Predicate_Attributes): Indicate what is propagated
	and remove ??? comment.
	* sem_util.adb (Cannot_Raise_Constraint_Error): Remove unneeded
	test of Do_Tag_Check.
	(Is_Local_Variable_Reference): Extend function to testing for
	formals of mode out as well.
	* sinfo.ads: Remove ??? comment about flag
	Convert_To_Return_False indicating that the flag is obsolete (in
	fact it's used). Remove references to Do_Accessibility_Check and
	Do_Tag_Check (and the two associated ??? comments), as these
	flags are unneeded.
	* sinfo-cn.adb (Change_Conversion_To_Unchecked): Remove call of
	Set_Do_Tag_Check (N, False).
	* targparm.ads (Support_Atomic_Primitives_On_Target): Remove ???
	comment, plus minor reformatting.