Digit separators mess with comments

To reproduce, try something like
```tex
\begin{codeblock}
unsigned x = 0xffff'ffff; // comment
\end{codeblock}
```

This will treat `// comment` as having code font, presumably because the C++ parser is not aware of digit separators, so it thinks that `'ffff; // comment` is one long character literal with no closing `'`.