This check aims to address a relatively common benign error where
Objective-C subclass initializers call -self on their superclass instead
of invoking a superclass initializer, typically -init. The error is
typically benign because libobjc recognizes that improper initializer
chaining is common¹.
One theory for the frequency of this error might be that -init and -self
have the same return type which could potentially cause inappropriate
autocompletion to -self instead of -init. The equal selector lengths and
triviality of common initializer code probably contribute to errors like
this slipping through code review undetected.
This check aims to flag errors of this form in the interests of
correctness and reduce incidence of initialization failing to chain to
 "In practice, it will be hard to rely on this function. Many classes do not properly chain -init calls."
From _objc_rootInit in https://opensource.apple.com/source/objc4/objc4-750.1/runtime/NSObject.mm.auto.html.
Verified via make check-clang-tools.