This is necessary to resolve an MSVC compile error in capability.h:

struct RawSchema;
extern const RawSchema NULL_INTERFACE_SCHEMA;
constexpr RawSchema const* schema = &NULL_INTERFACE_SCHEMA;

causes MSVC to complain that `schema` is not initialized by a constant
expression. Providing the full definition of RawSchema resolves this
spurious error, so I moved it out of generated-header-support.h and into
its own raw-schema.h.