Cop idea: Detect usages of defined? that will always be truthy

[Earlopain]

Is your feature request related to a problem? Please describe.

defined? is a bit of a weird one, both in what it accept and in what it returns.

Usually it is used to check if a constant is defined via defined?(FooBar). But if the user is not very familiar with it they might write it as defined?("FooBar") or defined?(:FooBar) instead in order to not raise an exception during runtime when Ruby seemingly tries to access the constant. Ruby handles this of course. In fact you can write anything in there as long as it is not a syntax error and it will produce some kind of result.

Examples:

• /~https://github.com/sidekiq/sidekiq/pull/6474/files#diff-ebb94e42d8a2439ff96291e25860212cec96b8d9372a767a4f673255ea2c15c0R19
• /~https://github.com/sidekiq/sidekiq/pull/6430/files#diff-ebb94e42d8a2439ff96291e25860212cec96b8d9372a767a4f673255ea2c15c0R20
• /~https://github.com/ruby-amqp/kicks/pull/19/files#diff-f2f6997940a8dd633e0f0a95b2e66c427d578ddb62983a86523f253a17cb6a1dR5
• /~https://github.com/ruby-amqp/kicks/pull/12/files#diff-f2f6997940a8dd633e0f0a95b2e66c427d578ddb62983a86523f253a17cb6a1dR4

# bad
defined?("FooBar") # will always return "expression"
defined?(:FooBar) # will also always return "expression"
defined?("foo#{bar}") # you might have guessed it, also "expression"

# good - anything else
defined?(FooBar)
defined?(foo_bar)
defined?([foo, bar]) # ruby checks each entry for existance. You probably shouldn't write this as well but :shrug:
# etc. etc.