[SPARK-27117][SQL] current_date/current_timestamp should not refer to columns with ansi parser mode

## What changes were proposed in this pull request?

This PR is a followup of https://github.com/apache/spark/pull/19559 .

It revisits https://issues.apache.org/jira/browse/SPARK-27117 , which should be an invalid use case according to the SQL standard.

`current_date/current_timestamp` are reserved keywords, if users want to access columns named `current_date/current_timestamp`, they should quote the name like ```select `current_date` from tbl```

If ansi mode is not enabled(which is the default), this PR won't introduce any changes.

## How was this patch tested?

a new test case

Closes #24039 from cloud-fan/current_datetime.

Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

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

@@ -603,7 +603,8 @@ valueExpression
     ;
 
 primaryExpression
-    : CASE whenClause+ (ELSE elseExpression=expression)? END                                   #searchedCase
+    : name=(CURRENT_DATE | CURRENT_TIMESTAMP)                                                  #currentDatetime
+    | CASE whenClause+ (ELSE elseExpression=expression)? END                                   #searchedCase
     | CASE value=expression whenClause+ (ELSE elseExpression=expression)? END                  #simpleCase
     | CAST '(' expression AS dataType ')'                                                      #cast
     | STRUCT '(' (argument+=namedExpression (',' argument+=namedExpression)*)? ')'             #struct

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

@@ -1212,6 +1212,21 @@ class AstBuilder(conf: SQLConf) extends SqlBaseBaseVisitor[AnyRef] with Logging
     }
   }
 
+  override def visitCurrentDatetime(ctx: CurrentDatetimeContext): Expression = withOrigin(ctx) {
+    if (conf.ansiParserEnabled) {
+      ctx.name.getType match {
+        case SqlBaseParser.CURRENT_DATE =>
+          CurrentDate()
+        case SqlBaseParser.CURRENT_TIMESTAMP =>
+          CurrentTimestamp()
+      }
+    } else {
+      // If the parser is not in ansi mode, we should return `UnresolvedAttribute`, in case there
+      // are columns named `CURRENT_DATE` or `CURRENT_TIMESTAMP`.
+      UnresolvedAttribute.quoted(ctx.name.getText)
+    }
+  }
+
   /**
    * Create a [[Cast]] expression.
    */

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

@@ -133,7 +133,7 @@ class CatalystSqlParser(conf: SQLConf) extends AbstractSqlParser {
 
 /** For test-only. */
 object CatalystSqlParser extends AbstractSqlParser {
-  val astBuilder = new AstBuilder(new SQLConf())
+  val astBuilder = new AstBuilder(SQLConf.get)
 }
 
 /**

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/parser/ExpressionParserSuite.scala

@@ -722,4 +722,16 @@ class ExpressionParserSuite extends PlanTest {
       }
     }
   }
+
+  test("current date/timestamp braceless expressions") {
+    withSQLConf(SQLConf.ANSI_SQL_PARSER.key -> "true") {
+      assertEqual("current_date", CurrentDate())
+      assertEqual("current_timestamp", CurrentTimestamp())
+    }
+
+    withSQLConf(SQLConf.ANSI_SQL_PARSER.key -> "false") {
+      assertEqual("current_date", UnresolvedAttribute.quoted("current_date"))
+      assertEqual("current_timestamp", UnresolvedAttribute.quoted("current_timestamp"))
+    }
+  }
 }