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

This exposes _BitInt literal suffixes __wb and u__wb as an extension
in C++. There is a new Extension warning, and the tests are
essentially the same as the existing _BitInt literal tests for C but
with a few additional cases.

Fixes #85223

GitOrigin-RevId: ca1f1c957232b05d6529916bcd769ae1c57ff935
11 files changed