This pull request creates two isOrderable functions in RowOrdering that can be used to check whether a data type or a sequence of expressions can be used in sorting.

Author: Reynold Xin <rxin@databricks.com>

Closes #7880 from rxin/SPARK-9546 and squashes the following commits:

f9e322d [Reynold Xin] Fixed tests.
0439b43 [Reynold Xin] [SPARK-9546][SQL] Centralize orderable data type checking.