Definition and properties of the null reflection

[andralex]

The null reflection is useful for returning "not available" reflections and in a variety of other cases. It compares equal to itself and is different from any other reflection.

[brevzin]

Things we'd need to do:

• [ ] Extend zero-initialize (https://eel.is/c++draft/dcl.init#def:zero-initialization) to say that for std::meta::info you get a null reflection.
• [ ] Extend the definition of default-initialize (https://eel.is/c++draft/dcl.init#def:default-initialization) to just say that std::meta::info is zero-initialized.
• [ ] Extend the definition of const-default-constructible (https://eel.is/c++draft/dcl.init#def:const-default-constructible) to include std::meta::info
• [x] Don't have to change value-initialize (https://eel.is/c++draft/dcl.init#def:value-initialization), it just forwards to zero-initialize already.
• [ ] Extend [expr.eq] to start by saying that if both operands are the null reflection, true. Otherwise if either is a null reflection, false.
• [ ] In [expr.refl] we'll need to at least add a sentence about how the null reflection is not a reflection of anything. Maybe copy off how we just introduce "null pointer value" in https://eel.is/c++draft/basic#compound-3