[C++] Function for casting from days since UNIX epoch to int64 date

[asfimport]

Follow to ARROW-605/ARROW-452

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• [C++] Type casting function kernel suite (is part of)

_{Note: This issue was originally created as ARROW-609. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
This was done in ARROW-1484: 54d5c81#diff-7dc816b87db253d3874e56fa9145159f