[clang] [Clang] Register __has_extension(bit_int) and __has_feature(bit_int) (PR #203871)

[Xavier Roche via cfe-commits]

xroche wrote:

Closing per philnik's point: `__BITINT_MAXWIDTH__` is the standard portable probe and the right path. The follow-up libcxx PR will migrate the existing `TEST_HAS_EXTENSION(bit_int)` test sites to `__BITINT_MAXWIDTH__` and fix the latent test bugs that have been hiding behind the dead probe.

https://github.com/llvm/llvm-project/pull/203871