flatbuffers: Missing std includes (cpp codegen)

Looks like flatc doesn’t include the necessary std includes for the code it generates. Soemtimes this is OK because flatbuffers.h is included and gets a few of them, but I’m getting compilation errors with --cpp-static-reflection because it uses std::array<> but never actually includes <array>.

Also looks like it uses std::declval<> without including <type_traits> and std::vector<> without including <vector>. I think the generated files should include everything they need to work properly and not rely on transient includes of stl via flatbuffers.h.

I am using v2.0.0, though I also tried HEAD (d959e232082641fb30dba457085c6bdc97419f4c at the time).