[SPARK-14769][SQL] Create built-in functionality for variable substitution

## What changes were proposed in this pull request?
In order to fully merge the Hive parser and the SQL parser, we'd need to support variable substitution in Spark. The implementation of the substitute algorithm is mostly copied from Hive, but I simplified the overall structure quite a bit and added more comprehensive test coverage.

Note that this pull request does not yet use this functionality anywhere.

## How was this patch tested?
Added VariableSubstitutionSuite for unit tests.

Author: Reynold Xin <rxin@databricks.com>

Closes #12538 from rxin/SPARK-14769.