ddl: Fix placement policy in alter table partition by (#48631)

[ti-chi-bot]

This is an automated cherry-pick of #48631

What problem does this PR solve?

Issue Number: close #48630

Problem Summary:
When specifying PLACEMENT POLICY per partitions in ALTER TABLE t PARTITION BY ... it failed, due to missed lookup of Name -> ID of the placement policy.
Workaround is to not assign placement policy and then alter the placement policy by:
https://docs.pingcap.com/tidb/dev/placement-rules-in-sql#specify-a-placement-policy-for-a-partitioned-table

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

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