Restrict contexts from which injected declarations may be produced

[katzdm]

P2996 stipulates certain circumstances under which an injected declaration (i.e., define_aggregate) may be produced.
https://brevzin.github.io/cpp_proposals/2996_reflection/p2996r8.html#pnum_112

These restrictions should be implemented and enforced. Probably also this will involve encoding the notion of "plainly constant-evaluated expression".