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/nbtree/nbtinsert.c

@@ -2043,9 +2043,10 @@ _bt_isequal(TupleDesc itupdesc, Page page, OffsetNumber offnum,
 		if (isNull || (scankey->sk_flags & SK_ISNULL))
 			return false;
 
-		result = DatumGetInt32(FunctionCall2(&scankey->sk_func,
-											 datum,
-											 scankey->sk_argument));
+		result = DatumGetInt32(FunctionCall2Coll(&scankey->sk_func,
+												 scankey->sk_collation,
+												 datum,
+												 scankey->sk_argument));
 
 		if (result != 0)
 			return false;