ARROW-7162: [C++] Cleanup warnings in cmake_modules/SetupCxxFlags.cmake
ARROW-7162: [C++] Cleanup warnings in cmake_modules/SetupCxxFlags.cmake

Closes #5828 from pitrou/ARROW-7162-cleanup-clang-warnings and squashes the following commits:

e6cb6cd2a <Antoine Pitrou> ARROW-7162:  Cleanup warnings in cmake_modules/SetupCxxFlags.cmake

Authored-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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