Support autocorrection for Lint/EnsureReturn

[koic]

This PR supports autocorrection for Lint/EnsureReturn.

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Before submitting the PR make sure the following are checked:

• 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.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

Looks good!

[marcandre]

Sadly, these autocorrections are not producing equivalent code. Removing the return like this can change the control flow within the ensure block:

ensure
  return foo if condition
  do_something_else
# can not be corrected to
ensure
  foo if condition
  do_something_else

More importantly, even when there are no exceptions raised any autocorrection changes the return value (as the result of the ensure block is ignored)

This autocorrection could also swallow user comments.

I don't know why, but this PR also stopped flagging returns with multiple values, but the argument against return in ensure is the same.

I'm reverting this in #8388