Make a safe detection for Performance/StringInclude

[koic]

This PR makes a safe detection for Performance/StringInclude cop when match? argument is a variable.
Symbol object does not have an include? method.

The following is an auto-corrected example code.

% ruby -e ':symbol.include?(/pattern/)'
Traceback (most recent call last):
-e:1:in `<main>': undefined method `include?' for :symbol:Symbol (NoMethodError)

A variable possible to be a symbol object, so if match? argument is a variable, accept it.

As an unsafe option it might be possible to provide detection when argument is a variable.
First of all, this PR makes detection safe.

This PR haven't added a changelog entry as it is an unreleased feature.

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