Move CLIENTHELLO_MSG up in the header

We'll be adding a field of this type to struct ssl_st in a subsequent
commit, and need the type definition to be in scope already.
Also move up the RAW_EXTENSION definition that it depends on.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2279)