[SPARK-20748][SQL] Add built-in SQL function CH[A]R.

## What changes were proposed in this pull request?
Add built-in SQL function `CH[A]R`:
For `CHR(bigint|double n)`, returns the ASCII character having the binary equivalent to `n`. If n is larger than 256 the result is equivalent to CHR(n % 256)

## How was this patch tested?
unit tests

Author: Yuming Wang <wgyumg@gmail.com>

Closes #18019 from wangyum/SPARK-20748.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala

@@ -276,6 +276,8 @@ object FunctionRegistry {
 
     // string functions
     expression[Ascii]("ascii"),
+    expression[Chr]("char"),
+    expression[Chr]("chr"),
     expression[Base64]("base64"),
     expression[Concat]("concat"),
     expression[ConcatWs]("concat_ws"),

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringExpressions.scala

@@ -1267,6 +1267,51 @@ case class Ascii(child: Expression) extends UnaryExpression with ImplicitCastInp
   }
 }
 
+/**
+ * Returns the ASCII character having the binary equivalent to n.
+ * If n is larger than 256 the result is equivalent to chr(n % 256)
+ */
+// scalastyle:off line.size.limit
+@ExpressionDescription(
+  usage = "_FUNC_(expr) - Returns the ASCII character having the binary equivalent to `expr`. If n is larger than 256 the result is equivalent to chr(n % 256)",
+  extended = """
+    Examples:
+      > SELECT _FUNC_(65);
+       A
+  """)
+// scalastyle:on line.size.limit
+case class Chr(child: Expression) extends UnaryExpression with ImplicitCastInputTypes {
+
+  override def dataType: DataType = StringType
+  override def inputTypes: Seq[DataType] = Seq(LongType)
+
+  protected override def nullSafeEval(lon: Any): Any = {
+    val longVal = lon.asInstanceOf[Long]
+    if (longVal < 0) {
+      UTF8String.EMPTY_UTF8
+    } else if ((longVal & 0xFF) == 0) {
+      UTF8String.fromString(Character.MIN_VALUE.toString)
+    } else {
+      UTF8String.fromString((longVal & 0xFF).toChar.toString)
+    }
+  }
+
+  override def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
+    nullSafeCodeGen(ctx, ev, lon => {
+      s"""
+        if ($lon < 0) {
+          ${ev.value} = UTF8String.EMPTY_UTF8;
+        } else if (($lon & 0xFF) == 0) {
+          ${ev.value} = UTF8String.fromString(String.valueOf(Character.MIN_VALUE));
+        } else {
+          char c = (char)($lon & 0xFF);
+          ${ev.value} = UTF8String.fromString(String.valueOf(c));
+        }
+      """
+    })
+  }
+}
+
 /**
  * Converts the argument from binary to a base 64 string.
  */

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/StringExpressionsSuite.scala

@@ -263,6 +263,19 @@ class StringExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
     checkEvaluation(Ascii(Literal.create(null, StringType)), null, create_row("abdef"))
   }
 
+  test("string for ascii") {
+    val a = 'a.long.at(0)
+    checkEvaluation(Chr(Literal(48L)), "0", create_row("abdef"))
+    checkEvaluation(Chr(a), "a", create_row(97L))
+    checkEvaluation(Chr(a), "a", create_row(97L + 256L))
+    checkEvaluation(Chr(a), "", create_row(-9L))
+    checkEvaluation(Chr(a), Character.MIN_VALUE.toString, create_row(0L))
+    checkEvaluation(Chr(a), Character.MIN_VALUE.toString, create_row(256L))
+    checkEvaluation(Chr(a), null, create_row(null))
+    checkEvaluation(Chr(a), 149.toChar.toString, create_row(149L))
+    checkEvaluation(Chr(Literal.create(null, LongType)), null, create_row("abdef"))
+  }
+
   test("base64/unbase64 for string") {
     val a = 'a.string.at(0)
     val b = 'b.binary.at(0)