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ARROW-8443: [Gandiva][C++] Fix Trunc and Round output types.

- Changed trunc and round to honor output precision and scale.
- Make it a no-op if there is no change in output scale and rounding scale is positive.

Closes #6942 from praveenbingo/ARROW-8443-1 and squashes the following commits:

c94f60642 <Praveen> ARROW-8443:  Fix Trunc and Round output types.

Authored-by: Praveen <praveen@dremio.com>
Signed-off-by: Praveen <praveen@dremio.com>