[SPARK-14668][SQL] Move CurrentDatabase to Catalyst

## What changes were proposed in this pull request?

This PR moves `CurrentDatabase` from sql/hive package to sql/catalyst. It also adds the function description, which looks like the following.

```
scala> sqlContext.sql("describe function extended current_database").collect.foreach(println)
[Function: current_database]
[Class: org.apache.spark.sql.execution.command.CurrentDatabase]
[Usage: current_database() - Returns the current database.]
[Extended Usage:
> SELECT current_database()]
```

## How was this patch tested?
Existing tests

Author: Yin Huai <yhuai@databricks.com>

Closes #12424 from yhuai/SPARK-14668.

2 files changed, 7 insertions, 2 deletions

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkOptimizer.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkOptimizer.scala
index cbde777d98..8dfbba779d 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkOptimizer.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkOptimizer.scala
@@ -18,9 +18,14 @@
 package org.apache.spark.sql.execution
 
 import org.apache.spark.sql.ExperimentalMethods
+import org.apache.spark.sql.catalyst.CatalystConf
+import org.apache.spark.sql.catalyst.catalog.SessionCatalog
 import org.apache.spark.sql.catalyst.optimizer.Optimizer
 
-class SparkOptimizer(experimentalMethods: ExperimentalMethods) extends Optimizer {
+class SparkOptimizer(
+    conf: CatalystConf,
+    sessionCatalog: SessionCatalog,
+    experimentalMethods: ExperimentalMethods) extends Optimizer(conf, sessionCatalog) {
   override def batches: Seq[Batch] = super.batches :+ Batch(
     "User Provided Optimizers", FixedPoint(100), experimentalMethods.extraOptimizations: _*)
 }
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/internal/SessionState.scala b/sql/core/src/main/scala/org/apache/spark/sql/internal/SessionState.scala
index 69e3358d4e..10497e4fdf 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/internal/SessionState.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/internal/SessionState.scala
@@ -80,7 +80,7 @@ private[sql] class SessionState(ctx: SQLContext) {
   /**
    * Logical query plan optimizer.
    */
-  lazy val optimizer: Optimizer = new SparkOptimizer(experimentalMethods)
+  lazy val optimizer: Optimizer = new SparkOptimizer(conf, catalog, experimentalMethods)
 
   /**
    * Parser that extracts expressions, plans, table identifiers etc. from SQL texts.