pgsql: Use C99 designated designators in a couple of places

Use C99 designated designators in a couple of places

This makes the arrays somewhat easier to read.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://postgr.es/m/202601281204.sdxbr5qvpunk@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1eb09ed63a8d8063dc6bb75c8f31ec564bf35250

Modified Files
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src/backend/access/heap/heapam.c  | 47 ++++++++++++++++++++-------------------
src/backend/postmaster/bgworker.c | 18 ++++++++++-----
2 files changed, 36 insertions(+), 29 deletions(-)