adjust check syntax so it can show info before expansion completes
and sometimes when there are syntax errors raised

In more detail, rearrange online check syntax's internal plumbing
so that syntax objects can be handed to check syntax at any point
(via a special log message, see the changes to the docs)
not just when expansion is finished

Use this in TR to get check syntax information even when there is
a type error (and also get the check syntax information processing
started before type checking starts (altho this isn't running
in a separate place, so there is no new parallelism here))