[SPARK-11398] [SQL] unnecessary def dialectClassName in HiveContext, and...

[SPARK-11398] [SQL] unnecessary def dialectClassName in HiveContext, and misleading dialect conf at the start of spark-sql

1. def dialectClassName in HiveContext is unnecessary.
In HiveContext, if conf.dialect == "hiveql", getSQLDialect() will return new HiveQLDialect(this);
else it will use super.getSQLDialect(). Then in super.getSQLDialect(), it calls dialectClassName, which is overriden in HiveContext and still return super.dialectClassName.
So we'll never reach the code "classOf[HiveQLDialect].getCanonicalName" of def dialectClassName in HiveContext.

2. When we start bin/spark-sql, the default context is HiveContext, and the corresponding dialect is hiveql.
However, if we type "set spark.sql.dialect;", the result is "sql", which is inconsistent with the actual dialect and is misleading. For example, we can use sql like "create table" which is only allowed in hiveql, but this dialect conf shows it's "sql".
Although this problem will not cause any execution error, it's misleading to spark sql users. Therefore I think we should fix it.
In this pr, while procesing “set spark.sql.dialect” in SetCommand, I use "conf.dialect" instead of "getConf()" for the case of key == SQLConf.DIALECT.key, so that it will return the right dialect conf.

Author: Zhenhua Wang <wangzhenhua@huawei.com>

Closes #9349 from wzhfy/dialect.