Warning-suppression fixes we ought to back-patch

I looked through the buildfarm's compiler warnings on back branches,
which is something I rarely do, and noted a bunch of warnings that
we are not seeing on master. That's because of recent commits that
aimed to silence those warning types. Most of the noise could be
suppressed by back-patching these two changes:

0909380e4 Allow PG_PRINTF_ATTRIBUTE to be different in C and C++ code.

This only matters for builds that are combining gcc with clang++ or
vice versa, but we have a dozen or so BF animals that are like that,
and each one is spewing a hundred "unrecognized format function type"
warnings.

8f1791c61 Fix some cases of indirectly casting away const.

While we've done a lot of const-cleanliness work recently, this patch
should be enough to silence most of the cast-away-const warnings I'm
seeing in the BF back branches. Only "midge" is showing them at the
moment, but more people will be seeing them as gcc 16 gets more
widespread. (I'd probably skip the ecpg bits though, as they are
a bit more convoluted than the rest, so the risk/benefit ratio
seems poor.)

Both of these patches were applied only to master, but with the
expectation that we'd back-patch once they'd baked awhile. It's
been two-plus months which seems like enough. So barring objections,
I will back-patch them once the current release freeze is over.

regards, tom lane