ARROW-13961: [C++] Fix use of non-const references, declaration without initialization
ARROW-13961: [C++] Fix use of non-const references, declaration without initialization

Fix some style errors/warnings that came up in ARROW-13138.

Closes #11121 from lidavidm/arrow-13961

Authored-by: David Li <dc24f9cfc84fb1675ce65c8444a2c3e2f0e5f297@gmail.com>
Signed-off-by: David Li <dc24f9cfc84fb1675ce65c8444a2c3e2f0e5f297@gmail.com>

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