pgsql: Reorder two functions in inval.c

Reorder two functions in inval.c

This file separates public and static functions with a separator
comment, but two routines were not defined in a location reflecting
that, so reorder them.

Back-patch commit c2bdd2c5b1d48a7e39e1a8d5e1d90b731b53c4c9 to v15 - v16.
This avoids merge conflicts in the next commit, which modifies a
function this moved. Exclude v14, which is so different that the merge
conflict savings would be immaterial.

Author: Aleksander Alekseev <aleksander(at)timescale(dot)com>
Reviewed-by: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/CAJ7c6TMX2dd0g91UKvcC+CVygKQYJkKJq1+ZzT4rOK42+b53=w@mail.gmail.com
Backpatch-through: 15-16

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ed75434c45c3fe47ffcf6a7bff3563edd08e648e
Author: Michael Paquier <michael(at)paquier(dot)xyz>

Modified Files
--------------
src/backend/utils/cache/inval.c | 183 ++++++++++++++++++++--------------------
1 file changed, 91 insertions(+), 92 deletions(-)