Fix a false positive for Style/DoubleNegation

[koic]

This PR fixes the following false positive for Style/DoubleNegation when inside returned conditional clauses with Ruby 2.7's pattern matching.

The following is a reproduction case.

def foo?
  case condition
  in foo
    !!foo
  in bar
    !!bar
  else
    !!baz
  end
end

It is expected that an offense will not be registered, but in actually an offense will be registered.

% bundle exec rubocop --only Style/DoubleNegation
(snip)

Inspecting 2 files
C.

Offenses:

example.rb:4:5: C: [Correctable] Style/DoubleNegation: Avoid the use of double negation (!!).
    !!foo
    ^
example.rb:6:5: C: [Correctable] Style/DoubleNegation: Avoid the use of double negation (!!).
    !!bar
    ^

2 files inspected, 2 offenses detected, 2 offenses auto-correctable

This is a follow-up to the same issue as #10474 and requires RuboCop AST 1.7 to fix. Therefore it bumps RuboCop AST required to use rubocop/rubocop-ast#227 to 1.7 or higher.

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