The C literal suffix for _BitInt is 'wb' or 'WB' (also allows a 'u' modifier for unsigned). We expose _BitInt in C++ as an extension, but do NOT expose the suffix, as it is one that the C++ committee might choose to use something else.

We should NOT do a single underscore, as that is reserved for the user defined literals (https://eel.is/c++draft/usrlit.suffix).

However, that standardeeze allows double-underscores for use by the implementation, so we could implement this with double-underscores without problem!

So allow: 123__WB, etc.