Support C++20 using enum in C++ Code Model

[arBmind]

Right now QtCreator does not highlight and auto complete on C++20 using enum.

• properly highlight the code
• suggest enum names
• add remaining enum cases to switch

Some sample codes:

enum class Test { A, B };
struct X { using enum Test; };
int main() { using enum Test; }
int switchDemo(Test t) {
  switch (t) {
    using enum Test;
  case A: break;
  case B: break;
  }
}