Use third_party clang on Mac.

-Wno-undefined-var-template flag is unknown by Xcode's clang. And it is
better to use the same clang version that is used on linux.

BUG=

Review-Url: https://codereview.chromium.org/1926633004
Cr-Commit-Position: refs/heads/master@{#35867}

build/standalone.gypi

@@ -1039,6 +1039,13 @@
               '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']},