* fix(preprocessor): reject abstract method bodies and abstract private class methods
Two abstract-method rules Zend enforces at compile time were missing:
- an abstract method with a body was accepted and the body silently
dropped; Zend fatals with "Abstract function A::f() cannot contain
body" (applies to classes and traits alike, probed on 8.4.13)
- `abstract private function` in a class can never be implemented,
since private methods do not participate in overriding; Zend fatals
with "Abstract function A::f() cannot be declared private". Traits
keep accepting it (allowed since PHP 8.0: the consuming class
supplies the private implementation)
Zend reports the private-modifier error before the body error when both
apply; the checks are ordered to match.
* test(preprocessor): cover abstract method declaration rules
Alessio Giacobbe