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

src/backend/access/gist/gistxlog.c

@@ -81,7 +81,7 @@ gistRedoPageUpdateRecord(XLogReaderState *record)
 		char	   *begin;
 		char	   *data;
 		Size		datalen;
-		int			ninserted = 0;
+		int			ninserted PG_USED_FOR_ASSERTS_ONLY = 0;
 
 		data = begin = XLogRecGetBlockData(record, 0, &datalen);
 

src/backend/access/transam/xlog.c

@@ -1785,7 +1785,7 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
 	XLogRecPtr	NewPageEndPtr = InvalidXLogRecPtr;
 	XLogRecPtr	NewPageBeginPtr;
 	XLogPageHeader NewPage;
-	int			npages = 0;
+	int			npages pg_attribute_unused() = 0;
 
 	LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);
 

src/backend/parser/gram.y

@@ -864,6 +864,7 @@ parse_toplevel:
 			stmtmulti
 			{
 				pg_yyget_extra(yyscanner)->parsetree = $1;
+				(void) yynerrs;		/* suppress compiler warning */
 			}
 			| MODE_TYPE_NAME Typename
 			{

src/backend/utils/adt/array_typanalyze.c

@@ -218,7 +218,6 @@ compute_array_stats(VacAttrStats *stats, AnalyzeAttrFetchFunc fetchfunc,
 {
 	ArrayAnalyzeExtraData *extra_data;
 	int			num_mcelem;
-	int			null_cnt = 0;
 	int			null_elem_cnt = 0;
 	int			analyzed_rows = 0;
 
@@ -320,8 +319,7 @@ compute_array_stats(VacAttrStats *stats, AnalyzeAttrFetchFunc fetchfunc,
 		value = fetchfunc(stats, array_no, &isnull);
 		if (isnull)
 		{
-			/* array is null, just count that */
-			null_cnt++;
+			/* ignore arrays that are null overall */
 			continue;
 		}
 

src/backend/utils/adt/jsonpath_gram.y

@@ -113,6 +113,7 @@ result:
 										*result = palloc(sizeof(JsonPathParseResult));
 										(*result)->expr = $2;
 										(*result)->lax = $1;
+										(void) yynerrs;
 									}
 	| /* EMPTY */					{ *result = NULL; }
 	;

src/bin/pgbench/exprparse.y

@@ -80,7 +80,10 @@ static PgBenchExpr *make_case(yyscan_t yyscanner, PgBenchExprList *when_then_lis
 
 %%
 
-result: expr				{ expr_parse_result = $1; }
+result: expr				{
+								expr_parse_result = $1;
+								(void) yynerrs; /* suppress compiler warning */
+							}
 
 elist:						{ $$ = NULL; }
 	| expr					{ $$ = make_elist($1, NULL); }