tree-wide: mark set-but-not-used variables as unused to make LLVM happy

LLVM 13 introduced `-Wunused-but-set-variable` diagnostic flag, which
trips over some intentionally set-but-not-used variables or variables
attached to cleanup handlers with side effects (`_cleanup_umask_`,
`_cleanup_(notify_on_cleanup)`, `_cleanup_(restore_sigsetp)`, etc.):

```
../src/basic/process-util.c:1257:46: error: variable 'saved_ssp' set but not used [-Werror,-Wunused-but-set-variable]
        _cleanup_(restore_sigsetp) sigset_t *saved_ssp = NULL;
                                                     ^
                                                     1 error generated.
```

src/oom/oomd-manager.c

@@ -387,7 +387,7 @@ static void clear_candidate_hashmapp(Manager **m) {
 static int monitor_memory_pressure_contexts_handler(sd_event_source *s, uint64_t usec, void *userdata) {
         /* Don't want to use stale candidate data. Setting this will clear the candidate hashmap on return unless we
          * update the candidate data (in which case clear_candidates will be NULL). */
-        _cleanup_(clear_candidate_hashmapp) Manager *clear_candidates = userdata;
+        _unused_ _cleanup_(clear_candidate_hashmapp) Manager *clear_candidates = userdata;
         _cleanup_set_free_ Set *targets = NULL;
         bool in_post_action_delay = false;
         Manager *m = userdata;

src/oom/oomd.c

@@ -116,7 +116,7 @@ static int parse_argv(int argc, char *argv[]) {
 }
 
 static int run(int argc, char *argv[]) {
-        _cleanup_(notify_on_cleanup) const char *notify_msg = NULL;
+        _unused_ _cleanup_(notify_on_cleanup) const char *notify_msg = NULL;
         _cleanup_(manager_freep) Manager *m = NULL;
         _cleanup_free_ char *swap = NULL;
         unsigned long long s = 0;