[Gandiva] Remove usages of mutable reference out arguments

[asfimport]

I have noticed several usages of mutable reference out arguments, e.g. gandiva/regex_util.h. We should change these to conform to the style guide (out arguments as pointers)

Reporter: Wes McKinney / @wesm

_{Note: This issue was originally created as ARROW-3332. Please see the migration documentation for further details.}

[github-actions]

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[Reranko05]

@kou I made a scoped pass on RegexUtil, converting the mutable reference out arguments there to pointer out arguments and updating the related local call sites. It compiles cleanly on my side.

Would you prefer I look for additional files under this issue first, or would a PR for this RegexUtil slice be reasonable?

[kou]

Could you open a PR for easy to discuss?

[kou]

Issue resolved by pull request 49879
#49879