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/resolve/resolved-dns-cache.c

@@ -274,7 +274,7 @@ static int dns_cache_link_item(DnsCache *c, DnsCacheItem *i) {
 
         first = hashmap_get(c->by_key, i->key);
         if (first) {
-                _cleanup_(dns_resource_key_unrefp) DnsResourceKey *k = NULL;
+                _unused_ _cleanup_(dns_resource_key_unrefp) DnsResourceKey *k = NULL;
 
                 /* Keep a reference to the original key, while we manipulate the list. */
                 k = dns_resource_key_ref(first->key);

src/resolve/resolved-dns-query.c

@@ -165,7 +165,7 @@ static int dns_query_candidate_add_transaction(
 }
 
 static int dns_query_candidate_go(DnsQueryCandidate *c) {
-        _cleanup_(dns_query_candidate_unrefp) DnsQueryCandidate *keep_c = NULL;
+        _unused_ _cleanup_(dns_query_candidate_unrefp) DnsQueryCandidate *keep_c = NULL;
         DnsTransaction *t;
         int r;
         unsigned n = 0;

src/resolve/resolved.c

@@ -23,7 +23,7 @@
 
 static int run(int argc, char *argv[]) {
         _cleanup_(manager_freep) Manager *m = NULL;
-        _cleanup_(notify_on_cleanup) const char *notify_stop = NULL;
+        _unused_ _cleanup_(notify_on_cleanup) const char *notify_stop = NULL;
         int r;
 
         log_setup();