[Java] Add CharacterLiteral to CompileTimeConstantExpr.getStringValue

java/ql/test/library-tests/constants/constants/Values.java

@@ -88,5 +88,7 @@ class Values {
         int var_local = final_local; //42
         int var_param = notConstant; //Not constant
         int var_nonfinald_local = var_field; //Not constant
+        String concatinatedString = "a" + "b"; //ab
+        String concatinatedChar = "ab" + 'c'; //abc
     }
 }