bug#33163: Adding a " in c++-mode fills the whole file with red

[Konstantin Kharlamov <hi-angel@yandex.ru>]

This long-standing problem appears occasionally, and only disappears if you kill the buffer completely, then reopen the file. I never got to report it because I never could find steps to reproduce it… until now :) Nvm the non-sense code, I reduced testcase as far as I managed.

# Steps to reproduce:

1. Save the following text in `test.cpp`
     int main() {
             puts(It's");
         auto foo = []()-> void {  };
             puts("It's");
     }
2. Open `emacs -Q ./test.cpp`
3. Now, the text lacks a quote in the first `puts(It's");`. Add it so it looks like `puts("It's");`. Essentially, if you'd compile it with g++, it lacks any syntax error besides `puts` being undeclared.

# Expected

The line with `auto foo…` is not red.

# Actual

The line with `auto foo…` (in particular) becomes red.

# Additional information

Emacs version GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30) of 2018-10-01, build with -flto.