C++: Add a test for constexpr functions

cpp/ql/test/library-tests/functions/constexpr/constexpr.cpp

@@ -0,0 +1,24 @@
+
+constexpr int fun_constexpr();
+int fun_not_constexpr();
+
+constexpr int overloaded_fun(int i) {
+    return 5;
+}
+
+int overloaded_fun(float f) {
+    return 6;
+}
+
+template <typename T>
+constexpr int template_fun(T t) {
+    return overloaded_fun(t);
+}
+
+void caller(void) {
+    int i;
+    float f;
+    template_fun(i);
+    template_fun(f);
+}
+