[TypeDeclaration] Add NativeMethodReflection support on ReturnStrictTypeAnalyzer

[samsonasik]

For support:

-   function aa(DateTime $dateTime)
+   function aa(DateTime $dateTime): string
    {
        return $dateTime->format('Y-m-d');
    }

Both NativeFunctionReflection and NativeMethodReflection use getReturnType() instead of getNativeReturnType()

[samsonasik]

All checks have passed 🎉 @TomasVotruba I am merging it ;)