[Ada] Fix bug in inherited user-defined-literal aspects for tagged types

[Ada] Fix bug in inherited user-defined-literal aspects for tagged types

gcc/ada/

	* sem_res.adb (Resolve): Two separate fixes. In the case where
	Find_Aspect for a literal aspect returns the aspect for a
	different (ancestor) type, call Corresponding_Primitive_Op to
	get the right callee. In the case where a downward tagged type
	conversion appears to be needed, generate a null extension
	aggregate instead, as per Ada RM 3.4(27).
	* sem_util.ads, sem_util.adb: Add new Corresponding_Primitive_Op
	function. It maps a primitive op of a tagged type and a
	descendant type of that tagged type to the corresponding
	primitive op of the descendant type. The body of this function
	was written by Javier Miranda.