Fix Style/ClassAndModuleChildren compaction for oneliner children

[Morriar]

Style/ClassAndModuleChildren with EnforcedStyle: Compact doesn't handle definitions on a single line properly.

Consider this example:

class Foo
  class Bar; end
end

Before this PR, the autocorrect was producing code with a syntax error:

class Foo::Barend

After this PR, the autocorrect is producing the correct code:

class Foo::Bar; end

This also fixes an error that was happening when autocorrecting this code:

class Foo
  class Bar end
end

Which was raising a Parser::ClobberingError: Parser::Source::TreeRewriter detected clobbering error.

After this PR, the autocorrect works properly:

class Foo::Bar end

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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.

[bbatsov]

The changes look good, but your branch has to be rebased on top of the current master branch due to merge conflicts.

[koic]

The fix conflicted with #10826. Sorry for the late notice...