[clang] [BitInt] Expose a _BitInt literal suffix in C++ (PR #86586)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

Aside from a nit with clang-format commenting, LGTM! @Sirraide are you happy with the state of things (you've got a "request changes" for your review). @erichkeane are you happy as well?

https://github.com/llvm/llvm-project/pull/86586