Split TemplateType::isSubTypeOf() and TemplateType::isAcceptedBy()

[arnaud-lb]

TemplateType::isAcceptedBy() delegates to TemplateType::isSubTypeOf(). Because of this, bounds are compared with isSubTypeOf(), which gives undesirable results in the context of isAcceptedBy().

For instance, ArrayAccess->isSuperTypeOf(Traversable) being maybe causes (T of ArrayAccess)->accepts(Traversable) to be maybe as well.

In this PR I split the implementations of TemplateType::isSubTypeOf() and TemplateType::isAcceptedBy(), so that isAcceptedBy() can compare bounds with accepts().

This fixes phpstan/phpstan#6589

[ondrejmirtes]

Makes sense, let's 🚢 this :)

[arnaud-lb]

I've marked the PR as ready 🚢🚢🚢 However I'm not super confident about everything in this PR yet, and I'll be AFK for a few days, so I wouldn't mind if this was not merged immediately.

[ondrejmirtes]

The worst thing that can happen are infinite recursions. We're not gaining anything by making it sit here, there's at least a chance we will gather some feedback when it's merged and there's something wrong with it :)

In the worst case scenario it can be reverted. There won't be a release for a few days anyway so don't worry too much :)

Thank you.