Part of the work for #660

Seen in a class overriding \Symfony\Component\HttpKernel\Client (While performing the checks analyzeOverrideRealSignature) - The magic @method has no "real" type

A solution is to store whether this method is only defined as a magic method in a base class, and to convert this to an UnanalyzableException

Add a new warning type (PhanParamMagicSignatureMismatch?) for method declarations which override a method that are as @method, and have incompatible param/return types