[SPARK-8244] [SQL] string function: find in set

This PR is based on #7186 (just fix the conflict), thanks to tarekauel .

find_in_set(string str, string strList): int

Returns the first occurance of str in strList where strList is a comma-delimited string. Returns null if either argument is null. Returns 0 if the first argument contains any commas. For example, find_in_set('ab', 'abc,b,ab,c,def') returns 3.

Only add this to SQL, not DataFrame.

Closes #7186

Author: Tarek Auel <tarek.auel@googlemail.com>
Author: Davies Liu <davies@databricks.com>

Closes #7900 from davies/find_in_set and squashes the following commits:

4334209 [Davies Liu] Merge branch 'master' of github.com:apache/spark into find_in_set
8f00572 [Davies Liu] Merge branch 'master' of github.com:apache/spark into find_in_set
243ede4 [Tarek Auel] [SPARK-8244][SQL] hive compatibility
1aaf64e [Tarek Auel] [SPARK-8244][SQL] unit test fix
e4093a4 [Tarek Auel] [SPARK-8244][SQL] final modifier for COMMA_UTF8
0d05df5 [Tarek Auel] Merge branch 'master' into SPARK-8244
208d710 [Tarek Auel] [SPARK-8244] address comments & bug fix
71b2e69 [Tarek Auel] [SPARK-8244] find_in_set
66c7fda [Tarek Auel] Merge branch 'master' into SPARK-8244
61b8ca2 [Tarek Auel] [SPARK-8224] removed loop and split; use unsafe String comparison
4f75a65 [Tarek Auel] Merge branch 'master' into SPARK-8244
e3b20c8 [Tarek Auel] [SPARK-8244] added type check
1c2bbb7 [Tarek Auel] [SPARK-8244] findInSet