[Ada] Crash building VSS with compiler built with assertions

When a tagged type T has aspect String_Literal, a derived type defines a
null extension T2, and the context to resolve the use of an object of
type T2 where the string literal is applicable is a class-wide type the
frontend crashes trying to evaluate if the object is a null extension.
This problem does not reproduce when the compiler is built with
assertions disabled.

gcc/ada/

	* sem_ch6.adb (Find_Corresponding_Spec): Avoid calling
	Is_Null_Extension with a class-wide type entity.
	(Overrides_Visible_Function): Handle alias entities.
	* sem_res.adb (Has_Applicable_User_Defined_Literal): Conversion
	not needed if the result type of the call is class-wide or if
	the result type matches the context type.
	* sem_util.ads (Is_Null_Extension): Adding documentation.
	(Is_Null_Extension_Of): Adding documentation.
	* sem_util.adb (Is_Null_Extension): Adding assertion.
	(Is_Null_Extension_Of): Adding assertions.

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