[SPARK-8264][SQL]add substring_index function

[davies]

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

Closes #7533

[SparkQA]

Test build #39285 has finished for PR 7843 at commit 3ce7802.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class SubstringIndex(strExpr: Expression, delimExpr: Expression, countExpr: Expression)

[SparkQA]

Test build #39288 has finished for PR 7843 at commit 391347b.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class SubstringIndex(strExpr: Expression, delimExpr: Expression, countExpr: Expression)

[davies]

@rxin This is ready to review

[rxin]

we can remove some of these tests since here we are mostly testing whether things work end to end, rather than unit testing all corner cases.

[rxin]

@davies is the behavior to return the entire string if there is not enough number of occurrence of the delimiter?

LGTM other than this question.

[davies]

@rxin I think so.

[rxin]

Merging in master. When you submit your next patch for something else, just delete some of the tests in StringFunctionsSuite.