C++: Add constexpr support for functions

cpp/ql/src/semmle/code/cpp/Function.qll

@@ -103,6 +103,27 @@ class Function extends Declaration, ControlFlowNode, AccessHolder, @function {
     function_defaulted(underlyingElement(this))
   }
 
+  /**
+   * Holds if this function is declared to be `constexpr`.
+   */
+  predicate isDeclaredConstexpr() {
+    this.hasSpecifier("declared_constexpr")
+  }
+
+  /**
+   * Holds if this function is `constexpr`. Normally, this holds if and
+   * only if `isDeclaredConstexpr()` holds, but in some circumstances
+   * they differ. For example, with
+   * ```
+   * int f(int i) { return 6; }
+   * template <typename T> constexpr int g(T x) { return f(x); }
+   * ```
+   * `g<int>` is declared constexpr, but is not constexpr.
+   */
+  predicate isConstexpr() {
+    this.hasSpecifier("is_constexpr")
+  }
+
   /**
    * Holds if this function is declared with `__attribute__((naked))` or
    * `__declspec(naked)`.