Expose an API for calculating catcache hash values.

Now that cache invalidation callbacks get only a hash value, and not a
tuple TID (per commits 632ae6829f and
b5282aa893), the only way they can restrict
what they invalidate is to know what the hash values mean.  setrefs.c was
doing this via a hard-wired assumption but that seems pretty grotty, and
it'll only get worse as more cases come up.  So let's expose a calculation
function that takes the same parameters as SearchSysCache.  Per complaint
from Marko Kreen.

src/backend/utils/cache/syscache.c

@@ -1050,6 +1050,30 @@ SysCacheGetAttr(int cacheId, HeapTuple tup,
 						isNull);
 }
 
+/*
+ * GetSysCacheHashValue
+ *
+ * Get the hash value that would be used for a tuple in the specified cache
+ * with the given search keys.
+ *
+ * The reason for exposing this as part of the API is that the hash value is
+ * exposed in cache invalidation operations, so there are places outside the
+ * catcache code that need to be able to compute the hash values.
+ */
+uint32
+GetSysCacheHashValue(int cacheId,
+					 Datum key1,
+					 Datum key2,
+					 Datum key3,
+					 Datum key4)
+{
+	if (cacheId < 0 || cacheId >= SysCacheSize ||
+		!PointerIsValid(SysCache[cacheId]))
+		elog(ERROR, "invalid cache ID: %d", cacheId);
+
+	return GetCatCacheHashValue(SysCache[cacheId], key1, key2, key3, key4);
+}
+
 /*
  * List-search interface
  */