[SPARK-26424][SQL][FOLLOWUP] Fix DateFormatClass/UnixTime codegen

## What changes were proposed in this pull request?

This PR fixes the codegen bug introduced by #23358 .

- https://amplab.cs.berkeley.edu/jenkins/view/Spark%20QA%20Test%20(Dashboard)/job/spark-master-test-maven-hadoop-2.7-ubuntu-scala-2.11/158/

```
Line 44, Column 93: A method named "apply" is not declared in any enclosing class
nor any supertype, nor through a static import
```

## How was this patch tested?

Manual. `DateExpressionsSuite` should be passed with Scala-2.11.

Closes #23394 from dongjoon-hyun/SPARK-26424.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

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

@@ -571,7 +571,7 @@ case class DateFormatClass(left: Expression, right: Expression, timeZoneId: Opti
     val tz = ctx.addReferenceObj("timeZone", timeZone)
     val locale = ctx.addReferenceObj("locale", Locale.US)
     defineCodeGen(ctx, ev, (timestamp, format) => {
-      s"""UTF8String.fromString($tf.apply($format.toString(), $tz, $locale)
+      s"""UTF8String.fromString($tf$$.MODULE$$.apply($format.toString(), $tz, $locale)
           .format($timestamp))"""
     })
   }
@@ -741,11 +741,11 @@ abstract class UnixTime
       case StringType =>
         val tz = ctx.addReferenceObj("timeZone", timeZone)
         val locale = ctx.addReferenceObj("locale", Locale.US)
-        val dtu = TimestampFormatter.getClass.getName.stripSuffix("$")
+        val tf = TimestampFormatter.getClass.getName.stripSuffix("$")
         nullSafeCodeGen(ctx, ev, (string, format) => {
           s"""
             try {
-              ${ev.value} = $dtu.apply($format.toString(), $tz, $locale)
+              ${ev.value} = $tf$$.MODULE$$.apply($format.toString(), $tz, $locale)
                 .parse($string.toString()) / 1000000L;
             } catch (java.lang.IllegalArgumentException e) {
               ${ev.isNull} = true;