[SPARK-20948][SQL] Built-in SQL Function UnaryMinus/UnaryPositive support string type

## What changes were proposed in this pull request?

Built-in SQL Function UnaryMinus/UnaryPositive support string type, if it's string type, convert it to double type, after this PR:
```sql
spark-sql> select positive('-1.11'), negative('-1.11');
-1.11   1.11
spark-sql>
```

## How was this patch tested?

unit tests

Author: Yuming Wang <wgyumg@gmail.com>

Closes #18173 from wangyum/SPARK-20948.

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

@@ -362,6 +362,8 @@ object TypeCoercion {
       case Average(e @ StringType()) => Average(Cast(e, DoubleType))
       case StddevPop(e @ StringType()) => StddevPop(Cast(e, DoubleType))
       case StddevSamp(e @ StringType()) => StddevSamp(Cast(e, DoubleType))
+      case UnaryMinus(e @ StringType()) => UnaryMinus(Cast(e, DoubleType))
+      case UnaryPositive(e @ StringType()) => UnaryPositive(Cast(e, DoubleType))
       case VariancePop(e @ StringType()) => VariancePop(Cast(e, DoubleType))
       case VarianceSamp(e @ StringType()) => VarianceSamp(Cast(e, DoubleType))
       case Skewness(e @ StringType()) => Skewness(Cast(e, DoubleType))

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

@@ -56,7 +56,6 @@ class ExpressionTypeCheckingSuite extends SparkFunSuite {
   }
 
   test("check types for unary arithmetic") {
-    assertError(UnaryMinus('stringField), "(numeric or calendarinterval) type")
     assertError(BitwiseNot('stringField), "requires integral type")
   }
 

sql/core/src/test/resources/sql-tests/inputs/operators.sql

@@ -89,3 +89,6 @@ select OCTET_LENGTH('abc');
 
 -- abs
 select abs(-3.13), abs('-2.19');
+
+-- positive/negative
+select positive('-1.11'), positive(-1.11), negative('-1.11'), negative(-1.11);

sql/core/src/test/resources/sql-tests/results/operators.sql.out

@@ -460,3 +460,11 @@ select abs(-3.13), abs('-2.19')
 struct<abs(-3.13):decimal(3,2),abs(CAST(-2.19 AS DOUBLE)):double>
 -- !query 55 output
 3.13	2.19
+
+
+-- !query 55
+select positive('-1.11'), positive(-1.11), negative('-1.11'), negative(-1.11)
+-- !query 55 schema
+struct<(+ CAST(-1.11 AS DOUBLE)):double,(+ -1.11):decimal(3,2),(- CAST(-1.11 AS DOUBLE)):double,(- -1.11):decimal(3,2)>
+-- !query 55 output
+-1.11	-1.11	1.11	1.11