IMPALA-1424: abs() function retains input type

This patch modifies the abs() built-in function so that it
retains the type of the input argument for the return type
in the same way as Postgres does.

Change-Id: I1750237b85bedbc3ce9d52330ac4d458b0aada3a
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4980
Reviewed-by: Martin Grund <mgrund@cloudera.com>
Tested-by: jenkins