[Fix #6956] Prevent auto-correct confliction of Lint/Lambda and Lint/UnusedBlockArgument

[koic]

Fixes #6956.

This PR prevents auto-correct confliction of Lint/Lambda and Lint/UnusedBlockArgument.

The following is the reproduction procedure.

# example.rb
c = -> event do
  puts 'Hello world'
end

% rubocop example.rb -a --only Style/Lambda,Lint/UnusedBlockArgument

Before

A reserved word lambda is broken.

% git diff
diff --git a/example.rb b/example.rb
index 3445c6f..b8e24f4 100644
--- a/example.rb
+++ b/example.rb
@@ -1,3 +1,3 @@
-c = -> event do
+c = lambda_ do |_event|
   puts 'Hello world'
 end

After

It will be a valid code.

% git diff
diff --git a/example.rb b/example.rb
index 3445c6f..9c22b37 100644
--- a/example.rb
+++ b/example.rb
@@ -1,3 +1,3 @@
-c = -> event do
+c = lambda do |_event|
   puts 'Hello world'
 end

---

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.

[koic]

📝 I extracted to the method because it was warned by Metrics cops.

[bbatsov]

Thanks!