ada: New warning about noncomposing user-defined "="

ada: New warning about noncomposing user-defined "="

Print warning for a user-defined "=" that does not compose
as might be expected (i.e. is ignored for predefined "=" of
a containing record or array type). This warning is enabled by
-gnatw_q; we don't enable it by default because it generates
too many false positives. We also don't enable it via -gnatwa.

gcc/ada/

	* exp_ch4.adb
	(Expand_Array_Equality): Do not test Ltyp = Rtyp here, because
	that is necessarily true. Move assertion thereof to more general
	place.
	(Expand_Composite_Equality): Pass in Outer_Type, for use in
	warnings. Rename Typ to be Comp_Type, to more clearly distinguish
	it from Outer_Type. Print warning when appropriate.
	* exp_ch4.ads: Minor comment fix.
	* errout.ads: There is no such pragma as Warning_As_Pragma --
	Warning_As_Error must have been intended. Improve comment for ?x?.
	* exp_ch3.adb
	(Build_Untagged_Equality): Update comment to be accurate for more
	recent versions of Ada.
	* sem_case.adb
	(Choice_Analysis): Declare user-defined "=" functions as abstract.
	* sem_util.ads
	(Is_Bounded_String): Give RM reference in comment.
	* warnsw.ads, warnsw.adb
	(Warn_On_Ignored_Equality): Implement new warning switch -gnatw_q.
	* doc/gnat_ugn/building_executable_programs_with_gnat.rst:
	Document new warning switch.
	* gnat_ugn.texi: Regenerate.