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ARROW-6332: [Java][C++][Gandiva] Misc fixes for varwidth vector allocation.

- Allow users to specify only a value count when allocating
  var length vectors.
- Print expected and minimum lengths when rejecting bitmaps
  in Gandiva.

Closes #5179 from praveenbingo/ARROW-6332 and squashes the following commits:

3b0c06f26 <Pindikura Ravindra> change test name and fix assert
035938827 <Praveen> ARROW-6332:  Misc fixes for varwidth vector allocation.

Lead-authored-by: Praveen <praveen@dremio.com>
Co-authored-by: Pindikura Ravindra <ravindra@dremio.com>
Signed-off-by: Pindikura Ravindra <ravindra@dremio.com>