topformflat confused by C++14/C23 digit separators

[emaxx-google]

C++14 and C23 introduced single-quotation-mark as digit separators:

int x = 1'048'576;

The parser in topformflat doesn't understand this notation, misinterpreting this as a beginning of a string literal - which confuses all the parsing after that point. And this might lead to inserting invalid line breaks - for example, there:

int x = 1'000;
char c = '"';
int y = 1'000;
char* s = "some_##value";  // a line break gets added before `#` (it's misinterpreted as a preprocessor directive)

This impedes the performance of the lines passes on such programs, because the pass often has to fall back to the non-reformatted file, or at least as the pass performs on wrongly grouped lines.