pgsql: Use C99 designated initializers for some structs

Use C99 designated initializers for some structs

These are just a few particularly egregious cases that were hard to read
and write, and error prone because of many similar adjacent types.

Discussion: https://www.postgresql.org/message-id/flat/4c9f01be-9245-2148-b569-61a8562ef190%402ndquadrant.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/98afa68d935215e8ab2c7de7fb45972a03e3da26

Modified Files
--------------
src/backend/access/transam/xact.c | 23 +-
src/backend/catalog/heap.c | 98 ++++++--
src/bin/psql/tab-complete.c | 434 ++++++++++-------------------------
src/pl/plpython/plpy_cursorobject.c | 39 +---
src/pl/plpython/plpy_planobject.c | 37 +--
src/pl/plpython/plpy_plpymodule.c | 22 +-
src/pl/plpython/plpy_resultobject.c | 55 ++---
src/pl/plpython/plpy_subxactobject.c | 37 +--
8 files changed, 240 insertions(+), 505 deletions(-)