CPP: 'if constexpr' test cases.

Author: geoffw0

cpp/ql/test/query-tests/Best Practices/Likely Errors/EmptyBlock/empty_block.cpp

@@ -56,4 +56,29 @@ int f(int x) {
 
   // GOOD (has comment): [FALSE POSITIVE]
   if (x) {}  // comment
+
+  // GOOD
+  if (x) {
+    if constexpr(1) {
+      f();
+    }
+  }
+
+  // GOOD
+  if (x) {
+    if constexpr(0) {
+      f();
+    }
+  }
+
+  // GOOD
+  if (x) {
+    if constexpr(1) {
+      f();
+    } else {
+	  f();
+	}
+  }
+
+  return 1;
 }