Fix #77291: magic methods inherited from a trait may be ignored

Fix #77291: magic methods inherited from a trait may be ignored

When adding methods from a trait, we must not assume that a method name
with the same length as the name of the using class is either a PHP 4
style constructor, or not a magic method at all – it may well be
another magic method.

We mostly preserve the spirit of the optimization which caused this
regression, and avoid string comparisons for all method names which can
never be magic methods.