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

[Xavier Roche via cfe-commits]

xroche wrote:

> Why would we want this? There is `__BITINT_MAXWIDTH__` already, which is actually portable, since it's required by the standard.

And I realize my previous PRs used those undefined gating, my bad:
https://github.com/llvm/llvm-project/pull/203876

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