[SPARK-11849][SQL] Analyzer should replace current_date and current_timestamp with literals

We currently rely on the optimizer's constant folding to replace current_timestamp and current_date. However, this can still result in different values for different instances of current_timestamp/current_date if the optimizer is not running fast enough.

A better solution is to replace these functions in the analyzer in one shot.

Author: Reynold Xin <rxin@databricks.com>

Closes #9833 from rxin/SPARK-11849.