[Build] Use brew clang compiler on mac (#8704)

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

![Screenshot 2025-04-30 at 7 28
39 AM](https://github.com/user-attachments/assets/9fe577b6-1879-4bc8-b99c-cf9909565083)


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

Author: hughperkins

.github/workflows/scripts/ti_build/compiler.py

@@ -3,11 +3,13 @@
 # -- stdlib --
 from pathlib import Path
 import os
+from os.path import join
 import json
 import platform
 import shutil
 import tempfile
 import sys
+import subprocess
 
 # -- third party --
 # -- own --
@@ -17,14 +19,20 @@
 from .tinysh import powershell
 
 
+def grep(value, target):
+    for line in target.split("\n"):
+        if value in line:
+            return line
+
+
 # -- code --
 @banner("Setup Clang")
 def setup_clang(as_compiler=True) -> None:
     """
     Setup Clang.
     """
     u = platform.uname()
-    if u.system in ("Linux", "Darwin"):
+    if u.system == "Linux":
         for v in ("", "-14", "-13", "-12", "-11", "-10"):
             clang = shutil.which(f"clang{v}")
             if clang is not None:
@@ -34,7 +42,13 @@ def setup_clang(as_compiler=True) -> None:
         else:
             error("Could not find clang of any version")
             return
-
+    elif u.system == "Darwin":
+        brew_config = subprocess.check_output(["brew", "config"]).decode("utf-8")
+        print("brew_config", brew_config)
+        brew_prefix = grep("HOMEBREW_PREFIX", brew_config).split()[1]
+        print("brew_prefix", brew_prefix)
+        clang = join(brew_prefix, "opt", "llvm@15", "bin", "clang")
+        clangpp = join(brew_prefix, "opt", "llvm@15", "bin", "clang++")
     elif (u.system, u.machine) == ("Windows", "AMD64"):
         out = get_cache_home() / "clang-14-v2"
         url = "https://github.com/taichi-dev/taichi_assets/releases/download/llvm15/clang-14.0.6-win-complete.zip"