[meta.syn] Synchronize reflect_constant/reflect_object parameters with definition

[eisenwave]

Fixes #8244.

This synchronizes the function signature with the definition in https://eel.is/c++draft/meta.reflection.result#lib:reflect_constant

The paper author has confirmed that the CWG intent is for the parameter to be T value; the use of const T& in the synopsis is an editorial oversight.

[eisenwave]

Now looking at it, we also have at least one mismatch between the parameter names in the synopsis (value) and in the definitions (expr). That's a much less severe problem than function signature differences though.

[jensmaurer]

Can you rename the function parameter in the synopsis to "expr" as well?

Otherwise, looks good.

[tkoeppe]

Great, thanks!