[SPARK-24571][SQL] Support Char literals

[MaxGekk]

What changes were proposed in this pull request?

In the PR, I propose to automatically convert a Literal with Char type to a Literal of String type. Currently, the following code:

val df = Seq("Amsterdam", "San Francisco", "London").toDF("city")
df.where($"city".contains('o')).show(false)

fails with the exception:

Unsupported literal type class java.lang.Character o
java.lang.RuntimeException: Unsupported literal type class java.lang.Character o
at org.apache.spark.sql.catalyst.expressions.Literal$.apply(literals.scala:78)

The PR fixes this issue by converting char to string of length 1. I believe it makes sense to does not differentiate char and string(1) in a unified, multi-language data platform like Spark which supports languages like Python/R.

[SparkQA]

Test build #91961 has finished for PR 21578 at commit 4210146.

• This patch fails due to an unknown error code, -9.
• This patch merges cleanly.
• This patch adds no public classes.

[MaxGekk]

jenkins, retest this, please

[SparkQA]

Test build #91974 has finished for PR 21578 at commit 4210146.

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

[MaxGekk]

@rxin Please, review the PR.

[gatorsmile]

LGTM

[gatorsmile]

@MaxGekk Could you resolve the conflict and then I will merge it?

[SparkQA]

Test build #92099 has finished for PR 21578 at commit 450079d.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #92100 has finished for PR 21578 at commit 3926655.

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

[gengliangwang]

LGTM

[gatorsmile]

Thanks! Merged to master.