[SQL] [MINOR] correct semanticEquals logic

It's a follow up of https://github.com/apache/spark/pull/6173, for expressions like `Coalesce` that have a `Seq[Expression]`, when we do semantic equal check for it, we need to do semantic equal check for all of its children.
Also we can just use `Seq[(Expression, NamedExpression)]` instead of `Map[Expression, NamedExpression]` as we only search it with `find`.

chenghao-intel, I agree that we probably never knows `semanticEquals` in a general way, but I think we have done that in `TreeNode`, so we can use similar logic. Then we can handle something like `Coalesce(children: Seq[Expression])` correctly.

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #6261 from cloud-fan/tmp and squashes the following commits:

4daef88 [Wenchen Fan] address comments
dd8fbd9 [Wenchen Fan] correct semanticEquals

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