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/include/utils/catcache.h

@@ -174,6 +174,10 @@ extern HeapTuple SearchCatCache(CatCache *cache,
 			   Datum v3, Datum v4);
 extern void ReleaseCatCache(HeapTuple tuple);
 
+extern uint32 GetCatCacheHashValue(CatCache *cache,
+					 Datum v1, Datum v2,
+					 Datum v3, Datum v4);
+
 extern CatCList *SearchCatCacheList(CatCache *cache, int nkeys,
 				   Datum v1, Datum v2,
 				   Datum v3, Datum v4);