Silence warnings about unused global variables

Similar to commit "Remove unused global variables", silence
-Wunused-but-set-global warnings from clang 23 (included in -Wall).
These are variables that are used in assert-enabled builds only, so
annotate them with PG_USED_FOR_ASSERTS_ONLY.

src/backend/utils/cache/syscache.c

@@ -85,7 +85,7 @@ StaticAssertDecl(lengthof(cacheinfo) == SysCacheSize,
 
 static CatCache *SysCache[SysCacheSize];
 
-static bool CacheInitialized = false;
+static bool CacheInitialized PG_USED_FOR_ASSERTS_ONLY = false;
 
 /* Sorted array of OIDs of tables that have caches on them */
 static Oid	SysCacheRelationOid[SysCacheSize];

src/test/modules/test_resowner/test_resowner_many.c

@@ -47,7 +47,7 @@ typedef struct
  * Priority of last call to the release callback.
  * This is used to check that the resources are released in correct order.
  */
-static uint32 last_release_priority = 0;
+static uint32 last_release_priority PG_USED_FOR_ASSERTS_ONLY = 0;
 
 /* prototypes for local functions */
 static void ReleaseManyTestResource(Datum res);