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

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

Mop up assorted set-but-not-used warnings in the back branches.
This includes back-patching relevant fixes from commit 152c9f7b8
the rest of the way, but there are also several cases that did not
appear in HEAD. Some of those we'd fixed in a retail way but not
back-patched, and others I think just got rewritten out of existence
during nearby refactoring.

While here, also back-patch b1980f6d0 (PL/Tcl: Fix compiler warnings
with Tcl 8.6) into 9.2, so that that branch compiles warning-free
with modern Tcl.

Per 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 all the way to 9.2.

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

Branch
------
REL9_4_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/3c5a33a52e3a4126679fadd67d215032fdb9c65b

Modified Files
--------------
contrib/hstore/crc32.c | 6 ++----
contrib/ltree/crc32.c | 6 ++----
src/backend/access/transam/xlog.c | 4 ++++
src/backend/optimizer/util/var.c | 3 ---
src/backend/parser/gram.y | 1 +
src/backend/utils/adt/array_typanalyze.c | 4 +---
src/backend/utils/adt/varlena.c | 3 +--
7 files changed, 11 insertions(+), 16 deletions(-)