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ARROW-6841: [C++] Migrate to LLVM 8

We also use clang-format-8 and clang-tidy-8 too.

We use LLVM 8 for all places but we still support LLVM 7 for now.
Because some Ursabot based CI still uses LLVM 7.

Closes #6266 from naitoh/ARROW-6841

Lead-authored-by: NAITOH Jun <naitoh@gmail.com>
Co-authored-by: Sutou Kouhei <kou@clear-code.com>
Co-authored-by: Prudhvi Porandla <prudhvi.porandla@icloud.com>
Signed-off-by: Wes McKinney <wesm+git@apache.org>