Set MyProc->heldLocks in ProcSleep

Set MyProc->heldLocks in ProcSleep

Previously, ProcSleep()'s caller was responsible for setting
MyProc->heldLocks, and we had comments to remind about that.  But it
seems simpler to make ProcSleep() itself responsible for it.
ProcSleep() already set the other info about the lock its waiting for
(waitLock, waitProcLock and waitLockMode), so it is natural for it to
set heldLocks too.

Reviewed-by: Maxim Orlov
Discussion: https://www.postgresql.org/message-id/7c2090cd-a72a-4e34-afaa-6dd2ef31440e@iki.fi