parser: set UnionStmt and ShowStmt as readonly (#755)

[djshow832]

Conflicting files:
util.go
util_test.go

What problem does this PR solve?

Fix pingcap/tidb#15050.
UNION statement and SHOW statement are not treated as readonly statement. These statements are executed again in retrying transactions.

What is changed and how it works?

Set UnionStmt and ShowStmt as readonly.

Check List

Tests

• Unit test

Code changes

• Has exported function/method change

Side effects

N/A

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release note

Release Note

• Treat UNION statement and SHOW statement as readonly statements.

[codecov]

Codecov Report

Merging #756 into release-3.0 will increase coverage by 0.12%.
The diff coverage is 100%.

@@               Coverage Diff               @@
##           release-3.0     #756      +/-   ##
===============================================
+ Coverage        64.63%   64.76%   +0.12%     
===============================================
  Files               31       31              
  Lines            10892    10897       +5     
===============================================
+ Hits              7040     7057      +17     
+ Misses            3443     3431      -12     
  Partials           409      409

Impacted Files	Coverage Δ
ast/util.go	85.18% <100%> (+57.91%)	⬆️

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[djshow832]

/run-all-tests

[kennytm]

LGTM

[tiancaiamao]

LGTM