ada: Fix bogus error on conditional expression with only user-defined literals

ada: Fix bogus error on conditional expression with only user-defined literals

This implements the recursive resolution of conditional expressions whose
dependent expressions are (all) user-defined literals the same way it is
implemented for operators.

gcc/ada/

	* sem_res.adb (Has_Applicable_User_Defined_Literal): Make it clear
	that the predicate also checks the node itself.
	(Try_User_Defined_Literal): Move current implementation to...
	Deal only with literals, named numbers and conditional expressions
	whose dependent expressions are literals or named numbers.
	(Try_User_Defined_Literal_For_Operator): ...this.  Remove multiple
	return False statements and put a single one at the end.
	(Resolve): Call Try_User_Defined_Literal instead of directly
	Has_Applicable_User_Defined_Literal for all nodes.  Call
	Try_User_Defined_Literal_For_Operator for operator nodes.