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

[yhuai]

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

[rxin]

i'm afraid this is too convoluted -- why don't we just have this function not taking any arguments, and then in the optimizer (which should take a SessionState) replace it with a literal? similar to how we compute CurrentDate.

[andrewor14]

sure, but this is just moving code. We can clean that up later

[SparkQA]

Test build #55955 has finished for PR 12424 at commit 6387829.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #55953 has finished for PR 12424 at commit 7b01fd3.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #55971 has finished for PR 12424 at commit 269a5c0.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • abstract class Optimizer(
  • case class GetCurrentDatabase(sessionCatalog: SessionCatalog) extends Rule[LogicalPlan]
  • class SparkOptimizer(

[rxin]

This conflicts with mine - but I can redo mine #12434

[rxin]

Merging in master.