[SPARK-16836][SQL] Add support for CURRENT_DATE/CURRENT_TIMESTAMP literals

## What changes were proposed in this pull request?
In Spark 1.6 (with Hive support) we could use `CURRENT_DATE` and `CURRENT_TIMESTAMP` functions as literals (without adding braces), for example:
```SQL
select /* Spark 1.6: */ current_date, /* Spark 1.6  & Spark 2.0: */ current_date()
```
This was accidentally dropped in Spark 2.0. This PR reinstates this functionality.

## How was this patch tested?
Added a case to ExpressionParserSuite.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #14442 from hvanhovell/SPARK-16836.

1 files changed, 5 insertions, 0 deletions

diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/parser/ExpressionParserSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/parser/ExpressionParserSuite.scala
index e73592c7af..849d962128 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/parser/ExpressionParserSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/parser/ExpressionParserSuite.scala
@@ -502,4 +502,9 @@ class ExpressionParserSuite extends PlanTest {
     assertEqual("1 - f('o', o(bar))", Literal(1) - 'f.function("o", 'o.function('bar)))
     intercept("1 - f('o', o(bar)) hello * world", "mismatched input '*'")
   }
+
+  test("current date/timestamp braceless expressions") {
+    assertEqual("current_date", CurrentDate())
+    assertEqual("current_timestamp", CurrentTimestamp())
+  }
 }