pgsql: Suppress variable-set-but-not-used warnings from clang 15.

Suppress variable-set-but-not-used warnings from clang 15.

clang 15+ will issue a set-but-not-used warning when the only
use of a variable is in autoincrements (e.g., "foo++;").
That's perfectly sensible, but it detects a few more cases that
we'd not noticed before.  Silence the warnings with our usual
methods, such as PG_USED_FOR_ASSERTS_ONLY, or in one case by
actually removing a useless variable.

One thing that we can't nicely get rid of is that with %pure-parser,
Bison emits "yynerrs" as a local variable that falls foul of this
warning.  To silence those, I inserted "(void) yynerrs;" in the
top-level productions of affected grammars.

Per recently-established project policy, this is a candidate
for back-patching into out-of-support branches: it suppresses
annoying compiler warnings but changes no behavior.  Hence,
back-patch to 9.5, which is as far as these patches go without
issues.  (A preliminary check shows that the prior branches
need some other set-but-not-used cleanups too, so I'll leave
them for another day.)

Discussion: https://postgr.es/m/514615.1663615243@sss.pgh.pa.us

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/85c077f9e0229d5e626f66d1a10e17a95c3564ff

Modified Files
--------------
src/backend/access/gist/gistxlog.c       | 2 +-
src/backend/access/transam/xlog.c        | 2 +-
src/backend/parser/gram.y                | 1 +
src/backend/utils/adt/array_typanalyze.c | 4 +---
src/bin/pgbench/exprparse.y              | 5 ++++-
5 files changed, 8 insertions(+), 6 deletions(-)