C++: Fix FP in PointlessComparison due to preprocessor

Reported by an LGTM customer here: https://discuss.lgtm.com/t/2-false-positives-in-c-for-comparison-is-always-same/1943.

Even though the comparison is pointless in the preprocessor configuration in effect during extraction, it is not pointless in other preprocessor configurations. Similar to ExprHasNoEffect, we'll now exclude results in functions that contain preprocessor-excluded code. I factored the similar code already used in ExprHasNoEffect in a non-recursive version into Preprocessor.qll, leaving the recursive version in ExprHasNoEffect.ql. I believe the recursive version is too aggressive for PointerlessComparison, which does no interprocedural analysis.

cpp/ql/test/query-tests/Likely Bugs/Arithmetic/PointlessComparison/RegressionTests.cpp

@@ -66,3 +66,17 @@ int regression_test_01(unsigned long bb) {
     return 1;
   }
 }
+
+int containsIfDef(int x) {
+  int result = 0;
+  if (x > 0) {
+    result = 1;
+  }
+#if _CONDITION
+  if (x < 0) {
+    result = -1;
+  }
+#endif
+
+  return result >= 0;
+}