[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.

1 files changed, 0 insertions, 18 deletions

diff --git a/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala b/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala
index ff93bfc4a3..7ebdad1a68 100644
--- a/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala
+++ b/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala
@@ -61,19 +61,6 @@ import org.apache.spark.unsafe.types.UTF8String
 import org.apache.spark.util.Utils
 
 /**
- * Returns the current database of metadataHive.
- */
-private[hive] case class CurrentDatabase(ctx: HiveContext)
-  extends LeafExpression with CodegenFallback {
-  override def dataType: DataType = StringType
-  override def foldable: Boolean = true
-  override def nullable: Boolean = false
-  override def eval(input: InternalRow): Any = {
-    UTF8String.fromString(ctx.sessionState.catalog.getCurrentDatabase)
-  }
-}
-
-/**
  * An instance of the Spark SQL execution engine that integrates with data stored in Hive.
  * Configuration for Hive is read from hive-site.xml on the classpath.
  *
@@ -133,11 +120,6 @@ class HiveContext private[hive](
   @transient
   protected[sql] override lazy val sessionState = new HiveSessionState(self)
 
-  // The Hive UDF current_database() is foldable, will be evaluated by optimizer,
-  // but the optimizer can't access the SessionState of metadataHive.
-  sessionState.functionRegistry.registerFunction(
-    "current_database", (e: Seq[Expression]) => new CurrentDatabase(self))
-
   /**
    * When true, enables an experimental feature where metastore tables that use the parquet SerDe
    * are automatically converted to use the Spark SQL parquet table scan, instead of the Hive