Add pg_nodiscard function declaration specifier
pg_nodiscard means the compiler should warn if the result of a
function call is ignored. The name "nodiscard" is chosen in alignment
with (possibly future) C and C++ standards. It maps to the GCC
attribute warn_unused_result.
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/flat/
e3753562-99cd-b65f-5aca-
687dfd1ec2fc@2ndquadrant.com