[LOW] Support C++14 integer literal digit separators

Filing this chiefly for the record. cppreference states that since C++14:

> Optional single quotes(') may be inserted between the digits as a separator. They are ignored by the compiler.

I verified this is true in gcc. At this time Wave does not understand them. For example:
```c++
#include

// can insert single quotes between digits starting in C++14:
int foo = 1'2'3; // wave thinks this is two ints and one char literal
int bar = 0x2'3; // wave thinks this is two ints and a single quote

int main() {
std::cout << foo << "\n"; // 123
std::cout << bar << "\n"; // 35

return 0;
}
```
I don't need this functionality but wanted to record it for some future maintainer.