pgsql: Add macros for looping through a List without a ListCell.

Add macros for looping through a List without a ListCell.

Many foreach loops only use the ListCell pointer to retrieve the
content of the cell, like so:

    ListCell   *lc;

    foreach(lc, mylist)
    {
        int         myint = lfirst_int(lc);

        ...
    }

This commit adds a few convenience macros that automatically
declare the loop variable and retrieve the current cell's contents.
This allows us to rewrite the previous loop like this:

    foreach_int(myint, mylist)
    {
        ...
    }

This commit also adjusts a few existing loops in order to add
coverage for the new/adjusted macros.  There is presently no plan
to bulk update all foreach loops, as that could introduce a
significant amount of back-patching pain.  Instead, these macros
are primarily intended for use in new code.

Author: Jelte Fennema-Nio
Reviewed-by: David Rowley, Alvaro Herrera, Vignesh C, Tom Lane
Discussion: https://postgr.es/m/CAGECzQSwXKnxGwW1_Q5JE%2B8Ja20kyAbhBHO04vVrQsLcDciwXA%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/14dd0f27d7cd56ffae9ecdbe324965073d01a9ff

Modified Files
--------------
src/backend/executor/execExpr.c             |  9 ++--
src/backend/replication/logical/relation.c  |  4 +-
src/backend/replication/logical/tablesync.c |  6 +--
src/backend/replication/pgoutput/pgoutput.c |  7 ++-
src/include/nodes/pg_list.h                 | 71 +++++++++++++++++++++++++----
5 files changed, 71 insertions(+), 26 deletions(-)