[libcxx-commits] [libcxx] [libc++] Replace __libcpp_{ctz, clz} with __builtin_{ctzg, clzg} (PR #133920)

[Peng Liu via libcxx-commits]

winner245 wrote:

The MacOS 13 CI is currently blocking this PR: 

```
error: use of undeclared identifier '__builtin_ctzg'
```


Based on the CI logs, the environment is using AppleClang 15 (with Xcode 15.2), which does not support `__builtin_{ctzg, clzg}` since these were introduced in Clang 19. 

```
  -- The C compiler identification is AppleClang 15.0.0.15000100
  -- The CXX compiler identification is AppleClang 15.0.0.15000100
  -- The ASM compiler identification is AppleClang
  -- Found assembler: /Applications/Xcode_15.2.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/cc
```

https://github.com/llvm/llvm-project/pull/133920