[ci] Move m1 job to use build with LLVM 15 (taichi-dev#6483)

Issue: taichi-dev#6447 

### Brief Summary
Just trying my luck in CI since my local mac is on macOS12.

.github/workflows/scripts/unix-build.sh

@@ -8,10 +8,17 @@ set -ex
 
 # TODO: Move llvm installation from container image to here
 if [[ "$LLVM_VERSION" == "15" ]]; then
+  if [[ $OSTYPE == "linux-"* ]]; then
     wget https://github.com/ailzhang/torchhub_example/releases/download/0.2/taichi-llvm-15-linux.zip
     unzip taichi-llvm-15-linux.zip && rm taichi-llvm-15-linux.zip
     export PATH="$PWD/taichi-llvm-15/bin:$PATH"
-    export TAICHI_CMAKE_ARGS="$TAICHI_CMAKE_ARGS -DTI_LLVM_15:BOOL=ON"
+  elif [[ $OSTYPE == "darwin"* ]]; then
+    # The following commands are done manually to save time.
+    #wget https://github.com/ailzhang/torchhub_example/releases/download/0.2/taichi-llvm-15-m1.zip
+    #unzip taichi-llvm-15-m1.zip && rm taichi-llvm-15-m1.zip
+    export PATH="/Users/github/taichi-llvm-15-m1/bin:$PATH"
+  fi
+  export TAICHI_CMAKE_ARGS="$TAICHI_CMAKE_ARGS -DTI_LLVM_15:BOOL=ON"
 fi
 
 build_taichi_wheel() {

.github/workflows/testing.yml

@@ -335,6 +335,7 @@ jobs:
           . .github/workflows/scripts/common-utils.sh
           prepare-build-cache
           brew install molten-vk
+          brew install llvm@15
           .github/workflows/scripts/unix-build.sh
         env:
           PY: ${{ matrix.python }}
@@ -346,6 +347,7 @@ jobs:
             -DTI_WITH_VULKAN:BOOL=ON
             -DTI_BUILD_TESTS:BOOL=ON
             -DTI_WITH_C_API=ON
+          LLVM_VERSION: 15
 
       - name: Test
         id: test

CMakeLists.txt

@@ -149,7 +149,7 @@ endmacro()
 
 if (APPLE)
   set(CLANG_OSX_FLAGS "-isysroot${CMAKE_OSX_SYSROOT}")
-  set(CLANG_HIGHEST_VERSION "12")
+  set(CLANG_HIGHEST_VERSION "13")
 else()
   if (TI_LLVM_15)
     set(CLANG_HIGHEST_VERSION "15")

taichi/runtime/cpu/jit_cpu.cpp

@@ -57,9 +57,10 @@ using namespace llvm::orc;
 #endif
 
 std::pair<JITTargetMachineBuilder, llvm::DataLayout> get_host_target_info() {
-#if defined(TI_PLATFORM_OSX) and defined(TI_ARCH_ARM)
+#if defined(TI_PLATFORM_OSX) and defined(TI_ARCH_ARM) and !defined(TI_LLVM_15)
   // JITTargetMachineBuilder::detectHost() doesn't seem to work properly on
   // Apple M1 yet. Hence the hardcoded strings here.
+  // Note this hack is no longer needed in LLVM15
   auto jtmb =
       JITTargetMachineBuilder(llvm::Triple("aarch64-apple-macosx11.0.0"));
   llvm::DataLayout data_layout("e-m:o-i64:64-i128:128-n32:64-S128");