[Ada] Removal of technical debt

[Ada] Removal of technical debt

gcc/ada/

	* exp_ch3.adb (Check_Missing_Others): Add comment.
	(Build_Initialization_Call): Remove inaccurate accessibility
	comment.
	* sem_ch12.adb (Analyze_Generic_Subprogram_Declaration): Remove
	test for Ada2012.
	(Analyze_Package_Instantiation): Remove speculative comment.
	(Inline_Instance_Body): Add comments for loops.
	(Build_Subprogram_Renaming): Remove comment about fix being
	partial and "ugly."
	(Instantiate_Subprogram_Body): Remove comment referencing DEC
	related internal issue.
	(Subtypes_Match): Add comment and simplify anonymous access
	test.
	(Is_Global): Add test for when E is an expanded name, and
	calculate the scope accordingly.
	* sem_ch6.adb (Analyze_Function_Return): Update comment
	regarding accessibility, and add check for
	Warn_On_Ada_2012_Compatibility.
	(Mask_Type_Refs): Add comments.
	(Analyze_Subprogram_Declaration): Remove mysterious suppression
	of elaboration checks.
	* sem_ch7.adb (Preserve_Full_Attributes): Preserve Is_Atomic
	value.
	* sem_ch8.adb (Most_Descendant_Use_Clause): Remove comment.
	(Note_Redundant_Use): Fix calls to Find_First_Use to be
	Find_Most_Prev.
	(Get_Object_Name): Modify error message to be more descriptive.
	(Known_But_Visible): Remove mysterious special case for
	GNAT_Mode.
	(Find_First_Use): Removed.
	(Find_Most_Prev): Renamed from Find_First_Use.
	* sem_prag.adb (Check_Static_Constraint): Add comments to
	routine.