[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.

sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4

@@ -500,6 +500,7 @@ valueExpression
 
 primaryExpression
     : constant                                                                                 #constantDefault
+    | name=(CURRENT_DATE | CURRENT_TIMESTAMP)                                                  #timeFunctionCall
     | ASTERISK                                                                                 #star
     | qualifiedName '.' ASTERISK                                                               #star
     | '(' expression (',' expression)+ ')'                                                     #rowConstructor
@@ -660,7 +661,7 @@ nonReserved
     | NULL | ORDER | OUTER | TABLE | TRUE | WITH | RLIKE
     | AND | CASE | CAST | DISTINCT | DIV | ELSE | END | FUNCTION | INTERVAL | MACRO | OR | STRATIFY | THEN
     | UNBOUNDED | WHEN
-    | DATABASE | SELECT | FROM | WHERE | HAVING | TO | TABLE | WITH | NOT
+    | DATABASE | SELECT | FROM | WHERE | HAVING | TO | TABLE | WITH | NOT | CURRENT_DATE | CURRENT_TIMESTAMP
     ;
 
 SELECT: 'SELECT';
@@ -880,6 +881,8 @@ OPTION: 'OPTION';
 ANTI: 'ANTI';
 LOCAL: 'LOCAL';
 INPATH: 'INPATH';
+CURRENT_DATE: 'CURRENT_DATE';
+CURRENT_TIMESTAMP: 'CURRENT_TIMESTAMP';
 
 STRING
     : '\'' ( ~('\''|'\\') | ('\\' .) )* '\''

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/parser/AstBuilder.scala

@@ -1022,6 +1022,19 @@ class AstBuilder extends SqlBaseBaseVisitor[AnyRef] with Logging {
     }
   }
 
+  /**
+   * Create a current timestamp/date expression. These are different from regular function because
+   * they do not require the user to specify braces when calling them.
+   */
+  override def visitTimeFunctionCall(ctx: TimeFunctionCallContext): Expression = withOrigin(ctx) {
+    ctx.name.getType match {
+      case SqlBaseParser.CURRENT_DATE =>
+        CurrentDate()
+      case SqlBaseParser.CURRENT_TIMESTAMP =>
+        CurrentTimestamp()
+    }
+  }
+
   /**
    * Create a function database (optional) and name pair.
    */

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())
+  }
 }

sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala

@@ -18,7 +18,7 @@
 package org.apache.spark.sql
 
 import java.math.MathContext
-import java.sql.Timestamp
+import java.sql.{Date, Timestamp}
 
 import org.apache.spark.{AccumulatorSuite, SparkException}
 import org.apache.spark.sql.catalyst.analysis.UnresolvedException
@@ -3017,4 +3017,13 @@ class SQLQuerySuite extends QueryTest with SharedSQLContext {
         data.selectExpr("`part.col1`", "`col.1`"))
     }
   }
+
+  test("current_date and current_timestamp literals") {
+    // NOTE that I am comparing the result of the literal with the result of the function call.
+    // This is done to prevent the test from failing because we are comparing a result to an out
+    // dated timestamp (quite likely) or date (very unlikely - but equally annoying).
+    checkAnswer(
+      sql("select current_date = current_date(), current_timestamp = current_timestamp()"),
+      Seq(Row(true, true)))
+  }
 }