[SPARK-17212][SQL] TypeCoercion supports widening conversion between DateType and TimestampType

## What changes were proposed in this pull request?

Currently, type-widening does not work between `TimestampType` and `DateType`.

This applies to `SetOperation`, `Union`, `In`, `CaseWhen`, `Greatest`,  `Leatest`, `CreateArray`, `CreateMap`, `Coalesce`, `NullIf`, `IfNull`, `Nvl` and `Nvl2`, .

This PR adds the support for widening `DateType` to `TimestampType` for them.

For a simple example,

**Before**

```scala
Seq(Tuple2(new Timestamp(0), new Date(0))).toDF("a", "b").selectExpr("greatest(a, b)").show()
```

shows below:

```
cannot resolve 'greatest(`a`, `b`)' due to data type mismatch: The expressions should all have the same type, got GREATEST(timestamp, date)
```

or union as below:

```scala
val a = Seq(Tuple1(new Timestamp(0))).toDF()
val b = Seq(Tuple1(new Date(0))).toDF()
a.union(b).show()
```

shows below:

```
Union can only be performed on tables with the compatible column types. DateType <> TimestampType at the first column of the second table;
```

**After**

```scala
Seq(Tuple2(new Timestamp(0), new Date(0))).toDF("a", "b").selectExpr("greatest(a, b)").show()
```

shows below:

```
+----------------------------------------------------+
|greatest(CAST(a AS TIMESTAMP), CAST(b AS TIMESTAMP))|
+----------------------------------------------------+
|                                1969-12-31 16:00:...|
+----------------------------------------------------+
```

or union as below:

```scala
val a = Seq(Tuple1(new Timestamp(0))).toDF()
val b = Seq(Tuple1(new Date(0))).toDF()
a.union(b).show()
```

shows below:

```
+--------------------+
|                  _1|
+--------------------+
|1969-12-31 16:00:...|
|1969-12-31 00:00:...|
+--------------------+
```

## How was this patch tested?

Unit tests in `TypeCoercionSuite`.

Author: hyukjinkwon <gurwls223@gmail.com>
Author: HyukjinKwon <gurwls223@gmail.com>

Closes #14786 from HyukjinKwon/SPARK-17212.

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

@@ -96,6 +96,9 @@ object TypeCoercion {
       val index = numericPrecedence.lastIndexWhere(t => t == t1 || t == t2)
       Some(numericPrecedence(index))
 
+    case (_: TimestampType, _: DateType) | (_: DateType, _: TimestampType) =>
+      Some(TimestampType)
+
     case _ => None
   }
 

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercionSuite.scala

@@ -188,6 +188,7 @@ class TypeCoercionSuite extends PlanTest {
     // TimestampType
     widenTest(NullType, TimestampType, Some(TimestampType))
     widenTest(TimestampType, TimestampType, Some(TimestampType))
+    widenTest(DateType, TimestampType, Some(TimestampType))
     widenTest(IntegerType, TimestampType, None)
     widenTest(StringType, TimestampType, None)