[clang] Revert "[X86][clang] Lift _BitInt() supported max width." (PR #81175)

[Harald van Dijk via cfe-commits]

hvdijk wrote:

That would be nice, but that will take time, and I did wait months already before creating this PR. Leaving Clang in its current state until someone steps up to fix it, in my opinion, does a massive disservice to users.

If code is written to use `_BitInt`, and it correctly uses configure or CMake checks to try to detect compiler support, and fall back to some other implementation if `_BitInt` is unavailable, such code would, with Clang, use the non-working implementation. In which case the configure or CMake check would end up amended to "and if Clang, don't bother". From experience, such checks remain for far too long after the original problem is fixed; leaving Clang in its current state, I suspect, ensures that once Clang behaves correctly, still `_BitInt` will not be used.

I'm happy if someone is willing to step up to fix the implementation, but I am not able to do it, and I would really like to avoid Clang 18 being released in this broken state, if possible, and I see no way short of a revert to realistically achieve that.

https://github.com/llvm/llvm-project/pull/81175