[SPARK-15199][SQL] Disallow Dropping Build-in Functions

#### What changes were proposed in this pull request?
As Hive and the major RDBMS behave, the built-in functions are not allowed to drop. In the current implementation, users can drop the built-in functions. However, after dropping the built-in functions, users are unable to add them back.

#### How was this patch tested?
Added a test case.

Author: gatorsmile <gatorsmile@gmail.com>

Closes #12975 from gatorsmile/dropBuildInFunction.

sql/core/src/main/scala/org/apache/spark/sql/execution/command/functions.scala

@@ -19,7 +19,7 @@ package org.apache.spark.sql.execution.command
 
 import org.apache.spark.sql.{AnalysisException, Row, SparkSession}
 import org.apache.spark.sql.catalyst.FunctionIdentifier
-import org.apache.spark.sql.catalyst.analysis.NoSuchFunctionException
+import org.apache.spark.sql.catalyst.analysis.{FunctionRegistry, NoSuchFunctionException}
 import org.apache.spark.sql.catalyst.catalog.CatalogFunction
 import org.apache.spark.sql.catalyst.expressions.{Attribute, ExpressionInfo}
 import org.apache.spark.sql.types.{StringType, StructField, StructType}
@@ -157,6 +157,9 @@ case class DropFunction(
         throw new AnalysisException(s"Specifying a database in DROP TEMPORARY FUNCTION " +
           s"is not allowed: '${databaseName.get}'")
       }
+      if (FunctionRegistry.builtin.functionExists(functionName)) {
+        throw new AnalysisException(s"Cannot drop native function '$functionName'")
+      }
       catalog.dropTempFunction(functionName, ifExists)
     } else {
       // We are dropping a permanent function.

sql/core/src/test/scala/org/apache/spark/sql/execution/command/DDLSuite.scala

@@ -927,6 +927,28 @@ class DDLSuite extends QueryTest with SharedSQLContext with BeforeAndAfterEach {
     }
   }
 
+  test("drop build-in function") {
+    Seq("true", "false").foreach { caseSensitive =>
+      withSQLConf(SQLConf.CASE_SENSITIVE.key -> caseSensitive) {
+        // partition to add already exists
+        var e = intercept[AnalysisException] {
+          sql("DROP TEMPORARY FUNCTION year")
+        }
+        assert(e.getMessage.contains("Cannot drop native function 'year'"))
+
+        e = intercept[AnalysisException] {
+          sql("DROP TEMPORARY FUNCTION YeAr")
+        }
+        assert(e.getMessage.contains("Cannot drop native function 'YeAr'"))
+
+        e = intercept[AnalysisException] {
+          sql("DROP TEMPORARY FUNCTION `YeAr`")
+        }
+        assert(e.getMessage.contains("Cannot drop native function 'YeAr'"))
+      }
+    }
+  }
+
   test("describe function") {
     checkAnswer(
       sql("DESCRIBE FUNCTION log"),