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
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/db8e36682d9508b7cf4eb1c69522fe7f4f805086

Modified Files
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src/backend/optimizer/util/var.c   | 3 ---
src/backend/utils/cache/relcache.c | 4 ----
2 files changed, 7 deletions(-)