pgsql: Move afterTriggerFiringDepth into AfterTriggersData

Move afterTriggerFiringDepth into AfterTriggersData

The static variable afterTriggerFiringDepth introduced by commit
5c54c3ed1b9 is logically part of the after-trigger state and in
hindsight should have been a field in AfterTriggersData alongside
query_depth and the other per-transaction after-trigger state.
Move it there as firing_depth.  Also update its comment to
accurately reflect its sole remaining purpose: signaling to
AfterTriggerIsActive() that after-trigger firing is active.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CA+HiwqFt4NGTNk7BinOsHHM48E9zGAa852vCfGoSe1bbL=JNFQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d6e96bacd3c05f1a2c1e350d04cc450e91311299

Modified Files
--------------
src/backend/commands/trigger.c | 36 ++++++++++++++++++------------------
1 file changed, 18 insertions(+), 18 deletions(-)