[SPARK-35764][SQL] Assign pretty names to TimestampWithoutTZType

### What changes were proposed in this pull request?

In the PR, I propose to override the typeName() method in TimestampWithoutTZType, and assign it a name according to the ANSI SQL standard
![image](https://user-images.githubusercontent.com/1097932/122013859-2cf50680-cdf1-11eb-9fcd-0ec1b59fb5c0.png)

### Why are the changes needed?

To improve Spark SQL user experience, and have readable types in error messages.

### Does this PR introduce _any_ user-facing change?

No, the new timestamp type is not released yet.
### How was this patch tested?

Unit test

Closes #32915 from gengliangwang/typename.

Authored-by: Gengliang Wang <gengliang@apache.org>
Signed-off-by: Max Gekk <max.gekk@gmail.com>

sql/catalyst/src/main/scala/org/apache/spark/sql/types/TimestampWithoutTZType.scala

@@ -48,6 +48,8 @@ class TimestampWithoutTZType private() extends AtomicType {
    */
   override def defaultSize: Int = 8
 
+  override def typeName: String = "timestamp without time zone"
+
   private[spark] override def asNullable: TimestampWithoutTZType = this
 }
 

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

@@ -1295,6 +1295,19 @@ abstract class AnsiCastSuiteBase extends CastSuiteBase {
       }
     }
   }
+
+  test("disallow type conversions between Numeric types and Timestamp without time zone type") {
+    import DataTypeTestUtils.numericTypes
+    checkInvalidCastFromNumericType(TimestampWithoutTZType)
+    var errorMsg = "cannot cast bigint to timestamp without time zone"
+    verifyCastFailure(cast(Literal(0L), TimestampWithoutTZType), Some(errorMsg))
+
+    val timestampWithoutTZLiteral = Literal.create(LocalDateTime.now(), TimestampWithoutTZType)
+    errorMsg = "cannot cast timestamp without time zone to"
+    numericTypes.foreach { numericType =>
+      verifyCastFailure(cast(timestampWithoutTZLiteral, numericType), Some(errorMsg))
+    }
+  }
 }
 
 /**