arrow

ARROW-15497: [C++][Homebrew] Use Clang Tools 12

We upgraded Clang Tools version to 12 from 8 in ARROW-14897.

Closes #12288 from kou/cpp-homebrew-llvm

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>