Fix subtly broken copy-on-write of ColumnLowCardinality dictionary

[al13n321]

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Fixed occasional LOGICAL_ERROR when using LowCardinality(String): "String offsets has data inconsistent with chars array".

Fixes #50715 (In addition to LOGICAL_ERRORs there were TSAN errors when it's enabled.)

This line in ColumnLowCardinality::forEachSubcolumn():

    callback(dictionary.getColumnUniquePtr());

was trying to pass a const reference to dictionary.column_unique to callback. But that's not what actually happened. getColumnUniquePtr() had slightly different return type than callback's argument type (ColumnPtr vs WrappedPtr), so this actually created a temporary local WrappedPtr and used a const reference to that.

That would be fine, if not for the fact that callback then const_casts the reference and assigns it. So it ended up assigning a temporary local variable instead of the field dictionary.column_unique. Oops.

This PR is a minimal fix that's easy to backport. Then #51072 does a little refactoring to prevent this specific error in future.

[robot-ch-test-poll1]

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