[clang] [clang]Implement the c23 stdc bit builtins (PR #185978)

[via cfe-commits]

enh-google wrote:

> > > Typically we don't bother to implement these unless libc starts using these, or needs them. Making builtins for these functions without the library needs them is kinda silly.
> > > DO we have a request for these coming from the libc maintainers?
> > 
> > 
> > I think they're important for two reasons: 1) we're going to want constexpr support for these for the same reason we want constexpr support for `strlen` in C so defining them as recognized library builtins is the way we do that, and 2) I think libc is going to want to have full support for bit-precise integer types and that's easier to support from a builtin currently. That said, CC @michaelrj-google for additional opinions
> 
> From the libc side having these builtins would be handy for both of the reasons Aaron mentioned. For optimization it would be helpful if the compiler could replace the libcall with a builtin, since things like `leading_zeros` can sometimes be reduced to a single instruction. Actually calling these bit functions as functions is unlikely to be optimal. CC: @enh-google

as i said in android's <stdbit.h>:
```
 * Ideally we'd have clang builtins for all of these, like gcc has.
 * If/when clang catches up, we can replace all of this with direct calls to
 * the relevant builtins.
```
so, yeah, "please subscribe me to your newsletter" --- i'll switch android's libc over to these as soon as they're available in the platform's clang :-)

see https://cs.android.com/android/platform/superproject/+/android-latest-release:bionic/libc/include/stdbit.h;l=2?q=file:stdbit.h for our current implementation, rewriting these in terms of existing builtins, which is mostly (but not always) fairly straightforward.

https://github.com/llvm/llvm-project/pull/185978