Pass collations to functions in FunctionCallInfoData, not FmgrInfo.

Since collation is effectively an argument, not a property of the function,
FmgrInfo is really the wrong place for it; and this becomes critical in
cases where a cached FmgrInfo is used for varying purposes that might need
different collation settings.  Fix by passing it in FunctionCallInfoData
instead.  In particular this allows a clean fix for bug #5970 (record_cmp
not working).  This requires touching a bit more code than the original
method, but nobody ever thought that collations would not be an invasive
patch...

src/backend/access/hash/hashutil.c

@@ -56,7 +56,8 @@ _hash_checkqual(IndexScanDesc scan, IndexTuple itup)
 		if (key->sk_flags & SK_ISNULL)
 			return false;
 
-		test = FunctionCall2(&key->sk_func, datum, key->sk_argument);
+		test = FunctionCall2Coll(&key->sk_func, key->sk_collation,
+								 datum, key->sk_argument);
 
 		if (!DatumGetBool(test))
 			return false;