pgsql: Introduce pg_attribute_nonnull(...)

Introduce pg_attribute_nonnull(...)

pg_attribute_nonnull(...) can be used to generate compiler warnings
when a function is called with the specified arguments set to NULL, as
per an idea from Andres Freund. An empty argument list indicates that
no pointer arguments can be NULL. pg_attribute_nonnull() only works for
compilers that support the nonnull function attribute. If nonnull is
not supported, pg_attribute_nonnull() has no effect.

As a beginning, this commit uses it for the DefineCustomXXXVariable()
functions to generate warnings when the "name" and "value" arguments are
set to NULL. This will likely be expanded to other places in the
future, where it makes sense.

Author: Nathan Bossart
Reviewed by: Michael Paquier, Tom Lane
Discussion: https://postgr.es/m/20220525061739.ur7x535vtzyzkmqo@alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/0507977aa4a356052ea0e5da209162e4b2125c1d

Modified Files
--------------
src/include/c.h | 11 +++++++++++
src/include/utils/guc.h | 10 +++++-----
2 files changed, 16 insertions(+), 5 deletions(-)