[Orc][RPC] Fix an obvious locking-order bug in RawByteChannel::startSendMessage.

The lock needs to be acquired before the data is sent, not afterwards. This
think-o slipped in during the refactor in r286620, but went unnoticed as the
resulting bug only manifests in multi-threaded clients (of which there are none
in-tree).

No unit test as the bug depends on thread scheduling.

llvm-svn: 291216