[BUG] The current definition of AST node Alias is ambiguous and easily confused

[LantaoJin]

What is the bug?
The current class definition of Alias looks

public class Alias extends UnresolvedExpression {

  /** Original field name. */
  private final String name;

  /** Expression aliased. */
  private final UnresolvedExpression delegated;

  /** Optional field alias. */
  private String alias;

When we need to build an Alias object, we actually use its name, for example:

      UnresolvedExpression aggExpression = internalVisitExpression(aggCtx.statsFunction());
      String name =
          aggCtx.alias == null
              ? getTextInQuery(aggCtx)
              : aggCtx.alias.getText();
      Alias alias = new Alias(name, aggExpression);
      aggListBuilder.add(alias);

But, when we need to use this alias name, sometimes we use its alias, for example:

        Expression arg = context.popNamedParseExpressions().get();
        return context.getNamedParseExpressions().push(
                org.apache.spark.sql.catalyst.expressions.Alias$.MODULE$.apply(arg,
                        node.getAlias() != null ? node.getAlias() : node.getName(),
                        NamedExpression.newExprId(),
                        seq(new java.util.ArrayList<String>()),
                        Option.empty(),

Although, there is no bug here since we check its nullable before using it, this still would be easy to introduce bugs in future.

What is the expected behavior?
Refactor this class definition to remove the alias member in Alias.

What is your host/environment?

• OS: [e.g. iOS]
• Version [e.g. 22]
• Plugins

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Do you have any additional context?
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