bug#56256: Emacs 28.1 gets stuck when typing some C++ code

[Iru Cai via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Emacs gets stuck when trying to type the '&' character at the end of the 
buffer when there's the following code in C++ mode (this code is from 
https://www.gingerbill.org/article/2015/08/19/defer-in-cpp/):

```

template <typename F>
struct privDefer {
     F f;
     privDefer(F f) : f(f) {}
     ~privDefer() { f(); }
};

template <typename F>
privDefer<F> defer_func(F f) {
     return privDefer<F>(f);
}

#define DEFER_1(x, y) x##y
#define DEFER_2(x, y) DEFER_1(x, y)
#define DEFER_3(x)    DEFER_2(x, __COUNTER__)
#define defer(code) auto DEFER_3(_defer_) = defer_func([

```

Steps to reproduce:

1. create a C++ source file with this code and open it with Emacs, if 
the file doesn't have a C++ source suffix, enable c++mode

2. type the '&' symbol at the last line after "defer_func(["

3. then Emacs gets stuck

I can see this bug in Emacs 28.1 and git revision 
5b1bb1af030597aab7f7895b6e3da9b430f9438a. I've also tried ``emacs -Q`` 
and ``emacs -Q -nw``, the bug still exists.