[SPARK-8264][SQL]add substring_index function

This PR is based on #7533 , thanks to zhichao-li

Closes #7533

Author: zhichao.li <zhichao.li@intel.com>
Author: Davies Liu <davies@databricks.com>

Closes #7843 from davies/str_index and squashes the following commits:

391347b [Davies Liu] add python api
3ce7802 [Davies Liu] fix substringIndex
f2d29a1 [Davies Liu] Merge branch 'master' of github.com:apache/spark into str_index
515519b [zhichao.li] add foldable and remove null checking
9546991 [zhichao.li] scala style
67c253a [zhichao.li] hide some apis and clean code
b19b013 [zhichao.li] add codegen and clean code
ac863e9 [zhichao.li] reduce the calling of numChars
12e108f [zhichao.li] refine unittest
d92951b [zhichao.li] add lastIndexOf
52d7b03 [zhichao.li] add substring_index function