[Build] Use brew clang compiler on mac

[hughperkins]

Issue: #

Brief Summary

Use brew clang compiler on mac

copilot:summary

Walkthrough

When building on latest Macs, e.g. Sequoia, the system clang is 16.0.0, or 17.0.0, depending on whether using XCode (16.0.0), or command line tools (17.0.0). Either way, the bytecode generated at python/taichi/_lib/runtime/runtime_arm64.bc is not loadable by llvm 15. Trying to load the clang-15/16 compiled runtime_arm64.bc with llvm 15 gives an error about unknown attribute (86). llvm 15 is the version of llvm used by taichi currently. therefore, building using system clang on macos sequoia prevents taichi from loading/running.

Example failure:

https://github.com/taichi-dev/taichi/actions/runs/14742813933/job/41384420135?pr=8688

To fix this, we use the clang from the brew installed llvm@15 instead. This then runs ok.

We assume in compile.py that brew has already been used earlier in the script to install llvm@15. We use brew config to locate brew, and then assume the clang path as {HOMEBREW_PREFIX}/opt/llvm@15/bin/clang, and similarly for clang++.

copilot:walkthrough

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[hughperkins]

In the M1 build logs, we can see this is correctly using the brew llvm15 clang 🙌 :

[hughperkins]

@feisuzhu thoughts on this? Anything you'd like me to check / test / modify / add / etc first, before we merge this? I think this is a first important step to being able to support building on Mac OS X Sequoia, 15.4.1., the latest/current version.

[bobcao3]

This looks correct, so... let's just merge! Thanks

[hughperkins]

Awesome, thank you 🙌