pgsql: Swap order of extern/static and pg_nodiscard

Swap order of extern/static and pg_nodiscard

When pg_nodiscard was first added, the C standard draft had it as a
function specifier, and so the code comment about placement was
written with that in mind. The final C23 standard has it as an
attribute and the placement rules are a bit different for that.
Specifically, it needs to be before extern or static. (Or at least
both current clang and gcc require that.) So just swap these. (To be
clear: The current implementation with gcc attributes doesn't care.
This change is just for maximum forward compatibility for non-gcc
compilers.) This also keeps the order consistent with the previously
introduced pg_noreturn. Also update the code comment to reflect the
mentioned developments since its introduction.

Reviewed-by: Dagfinn Ilmari Mannsåker <ilmari(at)ilmari(dot)org>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://www.postgresql.org/message-id/flat/pxr5b3z7jmkpenssra5zroxi7qzzp6eswuggokw64axmdixpnk(at)zbwxuq7gbbcw

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/05cbd6cb22d235d2e52ac309266a5fc50053dc16

Modified Files
--------------
src/include/c.h | 6 ++--
src/include/common/base64.h | 4 +--
src/include/nodes/pg_list.h | 68 +++++++++++++++++++++----------------------
src/include/storage/buffile.h | 2 +-
src/include/utils/guc.h | 2 +-
src/include/utils/palloc.h | 8 ++---
6 files changed, 45 insertions(+), 45 deletions(-)