CPP: Add a test of ConditionalDeclExpr.

cpp/ql/test/library-tests/exprs/conditional_decl/conditionaldeclexpr.cpp

@@ -0,0 +1,35 @@
+
+void do_something_with(bool b)
+{
+  // ...
+}
+
+void do_something_else_with(int i)
+{
+  // ...
+}
+
+void test_if(int x, int y)
+{
+  bool b = x < y;
+  do_something_with(b);
+
+  if (bool c = x < y) { // ConditionalDeclExpr
+    do_something_with(c);
+    x++;
+  }
+}
+
+void test_while(int x, int y)
+{
+  while (int d = x - y) { // ConditionalDeclExpr
+    do_something_else_with(d);
+  }
+}
+
+void test_for(int x, int y)
+{
+  for (int i = 0; bool c = x < y; x++) { // ConditionalDeclExpr
+    do_something_with(c);
+  }
+}