[SPARK-9401][SQL]Fully implement code generation for ConcatWs

[yjshen]

This PR adds full codegen support for ConcatWs, is a substitute of #7782

JIRA: https://issues.apache.org/jira/browse/SPARK-9401

cc @davies

[SparkQA]

Test build #41361 has finished for PR 8353 at commit affd497.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[davies]

Minor: j is only available in loop scope, so we don't need to create an unique name for it.

[davies]

LGTM, just one minor comment.

[davies]

@yjshen Another thing we could do is that we could count the number of strings, then create the array directly, do need to have a ArrayList.

[davies]

This could be the number of vargs that is UTF8String (${children.count(_ == StringType) - 1} ).

[yjshen]

I use this as the total num of strings, stringType + arrayType's element num.

[davies]

I means we can get rid of some many num ++

[yjshen]

ah, yes, I get this.

[yjshen]

@davies , more comments on this PR?

[SparkQA]

Test build #41403 has finished for PR 8353 at commit 84c1dfb.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #41404 has finished for PR 8353 at commit eb29b30.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[davies]

LGTM, merging this to master, thanks!