[SPARK-20931][SQL] ABS function support string type.

## What changes were proposed in this pull request?

ABS function support string type. Hive/MySQL support this feature.

Ref: https://github.com/apache/hive/blob/4ba713ccd85c3706d195aeef9476e6e6363f1c21/ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFAbs.java#L93

## How was this patch tested?
 unit tests

Author: Yuming Wang <wgyumg@gmail.com>

Closes #18153 from wangyum/SPARK-20931.

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

@@ -357,6 +357,7 @@ object TypeCoercion {
         val commonType = findCommonTypeForBinaryComparison(left.dataType, right.dataType).get
         p.makeCopy(Array(castExpr(left, commonType), castExpr(right, commonType)))
 
+      case Abs(e @ StringType()) => Abs(Cast(e, DoubleType))
       case Sum(e @ StringType()) => Sum(Cast(e, DoubleType))
       case Average(e @ StringType()) => Average(Cast(e, DoubleType))
       case StddevPop(e @ StringType()) => StddevPop(Cast(e, DoubleType))

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

@@ -57,7 +57,6 @@ class ExpressionTypeCheckingSuite extends SparkFunSuite {
 
   test("check types for unary arithmetic") {
     assertError(UnaryMinus('stringField), "(numeric or calendarinterval) type")
-    assertError(Abs('stringField), "requires numeric type")
     assertError(BitwiseNot('stringField), "requires integral type")
   }
 

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

@@ -85,3 +85,6 @@ select mod(7, 2), mod(7, 0), mod(0, 2), mod(7, null), mod(null, 2), mod(null, nu
 select BIT_LENGTH('abc');
 select CHAR_LENGTH('abc');
 select OCTET_LENGTH('abc');
+
+-- abs
+select abs(-3.13), abs('-2.19');

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

 -- Automatically generated by SQLQueryTestSuite
--- Number of queries: 54
+-- Number of queries: 55
 
 
 -- !query 0
@@ -444,3 +444,11 @@ select OCTET_LENGTH('abc')
 struct<octetlength(abc):int>
 -- !query 53 output
 3
+
+
+-- !query 54
+select abs(-3.13), abs('-2.19')
+-- !query 54 schema
+struct<abs(-3.13):decimal(3,2),abs(CAST(-2.19 AS DOUBLE)):double>
+-- !query 54 output
+3.13	2.19