[basic.types.general] Talk about "program points", not "certain contexts"

[eisenwave]

Especially after https://cplusplus.github.io/CWG/issues/3065.html, we use the wishy-washy language "in certain contexts" in https://eel.is/c++draft/basic.types.general#5:

A class type in certain contexts, an enumeration type in certain contexts ([dcl.enum]), or an array of unknown bound or of incomplete element type, is an incompletely-defined object type. [...]

It would be better to say "at certain program points" now that we have proper language to express this (I believe program points were recently introduced with C++26 Reflection).

I've floated the idea during yesterday's CWG telecon and it sounded like this may be doable editorially.