[SPARK-14004][SQL][MINOR] AttributeReference and Alias should only us…

…e the first qualifier to generate SQL strings

## What changes were proposed in this pull request?

Current implementations of `AttributeReference.sql` and `Alias.sql` joins all available qualifiers, which is logically wrong. But this implementation mistake doesn't cause any real SQL generation bugs though, since there is always at most one qualifier for any given `AttributeReference` or `Alias`.

This PR fixes this issue by only picking the first qualifiers.

## How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)

Existing tests should be enough.

Author: Cheng Lian <lian@databricks.com>

Closes apache#11820 from liancheng/spark-14004-single-qualifier.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/namedExpressions.scala

@@ -183,8 +183,7 @@ case class Alias(child: Expression, name: String)(
   }
 
   override def sql: String = {
-    val qualifiersString =
-      if (qualifiers.isEmpty) "" else qualifiers.map(quoteIdentifier).mkString("", ".", ".")
+    val qualifiersString = if (qualifiers.isEmpty) "" else qualifiers.head + "."
     s"${child.sql} AS $qualifiersString${quoteIdentifier(name)}"
   }
 }
@@ -299,8 +298,7 @@ case class AttributeReference(
   override def simpleString: String = s"$name#${exprId.id}: ${dataType.simpleString}"
 
   override def sql: String = {
-    val qualifiersString =
-      if (qualifiers.isEmpty) "" else qualifiers.map(quoteIdentifier).mkString("", ".", ".")
+    val qualifiersString = if (qualifiers.isEmpty) "" else qualifiers.head + "."
     s"$qualifiersString${quoteIdentifier(name)}"
   }
 }