[SPARK-8141] [SQL] Precompute datatypes for partition columns and reuse it

JIRA: https://issues.apache.org/jira/browse/SPARK-8141

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #6687 from viirya/reuse_partition_column_types and squashes the following commits:

dab0688 [Liang-Chi Hsieh] Reuse partitionColumnTypes.