[C++][Gandiva] Expression cache should not use ToString on data type

[asfimport]

The expression cache in gandiva generates uses the ToString() method of arrow::DataType() for both hashing and equality.

This is error-prone - we should have a visitor for generating hash, and use the equality visitor for comparison.

@fsaintjacques @praveenbingo  

Reporter: Pindikura Ravindra / @pravindra

_{Note: This issue was originally created as ARROW-5636. Please see the migration documentation for further details.}

[thisisnic]

This issue hasn't had activity in a long time. If it's still being worked on, please leave a comment. Otherwise, it will be closed on 23rd June.

Labelled Status: Stale-Warning for tracking.