[Ada] Fix check for looking for user defined literals

Recall that Has_Possible_Literal_Aspects only comes in if a given node
does not have any visible interpretation. If one operand is a literal we
assume that there may be a user-defined literal defined for some type to
be determined during the downward pass of resolution, but that depends
on the existence of some user-defined function or operator that can take
that literal as an actual. If analysis did not find such, the call is
illegal. I think that the following must be added to Operator_Check to
recover the proper error.

Note that this breaks current partial implementation of user-defined
literals.

gcc/ada/

	* sem_ch4.adb (Operator_Check): Fix condition.

gcc/ada/sem_ch4.adb

@@ -7260,7 +7260,9 @@ package body Sem_Ch4 is
             then
                return;
 
-            elsif Has_Possible_Literal_Aspects (N) then
+            elsif Present (Entity (N))
+              and then Has_Possible_Literal_Aspects (N)
+            then
                return;
 
             --  If we have a logical operator, one of whose operands is