ARROW-7353: [C++] Ignore -Wmissing-braces when building with clang
ARROW-7353: [C++] Ignore -Wmissing-braces when building with clang

This warning fails CHECKIN builds with clang (I observed it with Xcode 8.3.3), it's disabled in `-Wall` for gcc precisely due to spurious warnings with `std::array` https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25137

Closes #5995 from wesm/ARROW-7353 and squashes the following commits:

85b3effc5 <Wes McKinney> Ignore -Wmissing-braces in clang

Authored-by: Wes McKinney <3be70eed0275ec2606f002bff3514252e145d204@apache.org>
Signed-off-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>

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