tc_build/llvm: Handle the deprecation of CLANG_ENABLE_ARCMT

The CLANG_ENABLE_ARCMT option was deprecated in commit [1] due to the removal of ARCMigrate.
So check if the option exists before disabling it to avoid cmake warnings.

[1]: llvm/llvm-project@c4a0197

Signed-off-by: Dakkshesh <[email protected]>

Author: beakthoven

Diff for: tc_build/llvm.py

@@ -397,9 +397,6 @@ class LLVMSlimBuilder(LLVMBuilder):
     def configure(self):
         # yapf: disable
         slim_clang_defines = {
-            # Objective-C Automatic Reference Counting (we don't use Objective-C)
-            # https://clang.llvm.org/docs/AutomaticReferenceCounting.html
-            'CLANG_ENABLE_ARCMT': 'OFF',
             # We don't (currently) use the static analyzer and it saves cycles
             # according to Chromium OS:
             # https://crrev.com/44702077cc9b5185fc21e99485ee4f0507722f82
@@ -409,6 +406,16 @@ def configure(self):
             'CLANG_PLUGIN_SUPPORT': 'OFF',
         }
 
+        # Objective-C Automatic Reference Counting (we don't use Objective-C)
+        # https://clang.llvm.org/docs/AutomaticReferenceCounting.html
+        # Disable this option only if it exists to prevent CMake warnings.
+        # CLANG_ENABLE_ARCMT was deprecated in favor of CLANG_ENABLE_OBJC_REWRITER,
+        # which is disabled by default.
+        # https://github.com/llvm/llvm-project/commit/c4a019747c98ad9326a675d3cb5a70311ba170a2
+        arcmt_cmakelists = Path(self.folders.source, 'clang/lib/ARCMigrate/CMakeLists.txt')
+        if arcmt_cmakelists.exists():
+            slim_clang_defines['CLANG_ENABLE_ARCMT'] = 'OFF'
+
         llvm_build_runtime = self.cmake_defines.get('LLVM_BUILD_RUNTIME', 'ON') == 'ON'
         build_compiler_rt = self.project_is_enabled('compiler-rt') and llvm_build_runtime