Use third_party clang on Mac.

build/standalone.gypi

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 1021 matching lines @@
             'xcode_settings': {'GCC_TREAT_WARNINGS_AS_ERRORS': 'NO'},
           }, {
             'xcode_settings': {'GCC_TREAT_WARNINGS_AS_ERRORS': 'YES'},
           }],
           ['clang==1', {
             'xcode_settings': {
               'GCC_VERSION': 'com.apple.compilers.llvm.clang.1_0',
               'CLANG_CXX_LANGUAGE_STANDARD': 'c++11',  # -std=c++11
             },
             'conditions': [
+              ['clang_xcode==0', {
+                'xcode_settings': {
+                  'CC': '<(clang_dir)/bin/clang',
+                  'LDPLUSPLUS': '<(clang_dir)/bin/clang++',
+                  'CLANG_CXX_LIBRARY': 'libc++'
+                },
+              }],
               ['v8_target_arch=="x64" or v8_target_arch=="arm64" \
                 or v8_target_arch=="mips64el"', {
                 'xcode_settings': {'WARNING_CFLAGS': ['-Wshorten-64-to-32']},
               }],
             ],
           }],
         ],
         'target_conditions': [
           ['_type!="static_library"', {
             'xcode_settings': {'OTHER_LDFLAGS': ['-Wl,-search_paths_first']},
@@ skipping 393 matching lines @@
               '-fsanitize=cfi-vcall',
               '-fsanitize=cfi-derived-cast',
               '-fsanitize=cfi-unrelated-cast',
             ],
           }],
         ],
       },
     }],
   ],
 }