GTA V is coming to PC eventually. They've not 'officially confirmed it', but most people who know anything about Rockstar or previous GTA games knows it's coming and Rockstar isn't very good about hiding the fact that it is coming out. They won't admit to it, but it's obvious and they know that people know it's coming. They're just being 'coy' for marketing reasons.
They won't say anything because they're focusing on selling the console edition and officially announcing the eventual release of a PC version will hurt sales for the console versions a little. It won't make a huge impact of course, but it will have an impact so they're not going to admit to it until after the console version has been out for a bit.
Rockstar never ever releases GTA concurrently with the console editions. PC always gets an upgraded edition of the game several months later and GTA V will be no different.
So, yes. GTA V is coming out for PC, but probably not until several months after the console version, just like every other GTA game since GTA III.
They put out an add on their website recently hiring people to, and I quote, "Bring the latest titles to PC". [Hint: They're talking about porting GTA V].
So to answer half your question. Yes. GTA V is coming to PC eventually, but you're probably going to have to wait a while as it usually takes several months for Rockstar to release GTA after the console release. It will likely be worth the wait as the PC version has always included updated graphics and other new features as well and there's no reason to think GTA V will be any different.
As for Destiny. Yes...probably. Bungee is most likely doing the same thing as Rockstar and doesn't want to hurt console sales by announcing a PC version of their game right now. It won't be out until after the console versions anyway. There have been things said to that effect and Bungee had something up on their website about a PC version that was later removed.
It's in development, they're not talking about it, but yeah, it's probably coming too. The console versions will definitely be first though. Don't expect an announcement for a PC version before the console editions are released and have been out for a little while. They may say something before then, but I doubt it at this point.
The certainty for a PC release for both of these games is strong. The companies themselves aren't going to say anything as an announcement like that will impact the sales of the console editions. Sony and Microsoft probably have something to do with it as well as having GTA V and Destiny as 'exclusives' for a bit will help them move units as well. There has probably been some money paid to both game developers to keep a lid on their PC editions of these games.
You'll be able to eventually play both on PC, but expect to wait for both. Neither PC edition is likely to be released before next year, and probably not until spring or summer at that.