[SPARK-8010] [SQL] Promote types to StringType as implicit conversion in...

[SPARK-8010] [SQL] Promote types to StringType as implicit conversion in non-binary expression of HiveTypeCoercion

1. Given a query
`select coalesce(null, 1, '1') from dual` will cause exception:
java.lang.RuntimeException: Could not determine return type of Coalesce for IntegerType,StringType
2. Given a query:
`select case when true then 1 else '1' end from dual` will cause exception:
java.lang.RuntimeException: Types in CASE WHEN must be the same or coercible to a common type: StringType != IntegerType
I checked the code, the main cause is the HiveTypeCoercion doesn't do implicit convert when there is a IntegerType and StringType.

Numeric types can be promoted to string type

Hive will always do this implicit conversion.

Author: OopsOutOfMemory <victorshengli@126.com>

Closes #6551 from OopsOutOfMemory/pnts and squashes the following commits:

7a209d7 [OopsOutOfMemory] rebase master
6018613 [OopsOutOfMemory] convert function to method
4cd5618 [OopsOutOfMemory] limit the data type to primitive type
df365d2 [OopsOutOfMemory] refine
95cbd58 [OopsOutOfMemory] fix style
403809c [OopsOutOfMemory] promote non-string to string when can not found tighestCommonTypeOfTwo