[Ada] Tidy up implementation of Has_Compatible_Type

[Ada] Tidy up implementation of Has_Compatible_Type

gcc/ada/

	* sem_ch4.adb (Analyze_Membership_Op) <Find_Interpretation>: Handle
	both overloaded and non-overloaded cases.
	<Try_One_Interp>: Do a reversed call to Covers if the outcome of the
	call to Has_Compatible_Type is false.
	Simplify implementation after change to Find_Interpretation.
	(Analyze_User_Defined_Binary_Op): Be prepared for previous errors.
	(Find_Comparison_Types) <Try_One_Interp>: Do a reversed call to
	Covers if the outcome of the call to Has_Compatible_Type is false.
	(Find_Equality_Types) <Try_One_Interp>: Likewise.
	* sem_type.adb (Has_Compatible_Type): Remove the reversed calls to
	Covers.  Add explicit return on all paths.