[release-19.0] planner: support union statements with ctes (#15312)

[vitess-bot]

Description

This is a backport of #15312

[vitess-bot]

Review Checklist

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General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

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[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (b317589) 67.36% compared to head (19f0dcc) 67.35%.

Files	Patch %	Lines
go/vt/sqlparser/ast_format.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-19.0   #15324      +/-   ##
================================================
- Coverage         67.36%   67.35%   -0.01%     
================================================
  Files              1559     1559              
  Lines            192103   192105       +2     
================================================
- Hits             129402   129396       -6     
- Misses            62701    62709       +8     

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