ddl: move public hidden columns to a right position

[tangenta]

What problem does this PR solve?

Issue Number: close #110

Problem Summary:

• For onCreateIndex(), we can't update the hidden columns state step by step(e.g, from none to delete-only), because the non-public columns are ignored in the update assignments list construction(buildUpdateList). As a result, rebuildIndices() panic because the touched array does not contain the index(in delete-only state) columns. See ddl: fix expression index with insert causes losing index data pingcap/tidb#26248.
• We can't change the implementation of TableInfo.Cols() because "Columns[xxx.Offset]" is used everywhere. It is easy to make mistakes.
• We have to keep the assumption of "the public columns always come first in TableInfo.Columns". fix TestSchemaChangeForDropColumnsWithIndexes #45 and this PR follow this assumption.

What is changed and how it works?

Check List

Tests

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

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

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None