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GH-36182: [Gandiva][C++] Fix substring_index function when index is negative. (#36184)

### Rationale for this change

substring_index("Abc.DE.fGh", '.', -2) returns "fGh" but it should return "DE.fGh" (ie starting from the second occurrence of the delimiter from the right). The proposed behavior matches the behavior of other databases.

### What changes are included in this PR?

Fixed reverse index calculation and updated unit tests.

### Are these changes tested?

Yes, unit tests and integration testing.

### Are there any user-facing changes?

Function behavior change with substring_index function.

* Closes: #36182

Authored-by: Projjal Chanda <iam@pchanda.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>