[Fix #532] Fix false negatives for Performance/ChainArrayAllocation

[koic]

flat_map, filter_map, and collect_concat always return a new array, but they were missing from ALWAYS_RETURNS_NEW_ARRAY, so a chain built on them was only partially matched or not matched at all. nodes.flat_map(&:file_ids).compact.uniq reported one offense while the same-shaped map version reported two, and rewriting a flagged chain around flat_map silenced the cop without fixing the allocation it complained about.

Recognize the three methods as chain receivers. They are not added to HAS_MUTATION_ALTERNATIVE because no bang variants exist, so they are never suggested for ! conversion.

Fixes #532.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.