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ARROW-17770: [C++][Gandiva] Fix const correctness of Gandiva projector Evaluate (#14165)

I was trying to figure out the thread-safeness of Gandiva projector evaluation, i.e., whether I can use a single Projector to evaluate multiple inputs concurrently. I assumed it isn't safe because the Evaluate function is not marked const. However, as far as I understand, the Evaluate function merely executes a compiled function on the input, which doesn't modify a project's internal states and should be const.

Authored-by: Jin Shang <shangjin1997@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>