If the harassment has moved beyond the workplace, and someone is stalking you at home or wherever, then call the police.
As long as it's still at work, the easiest thing, and probably best choice for your career, is to find a new employer ASAP. We're talking about software engineers/devs here: this field is hot, and there's lots of demand. GTFO and find a better place to work. Just look at the women who's the subject of this article: she blew the whistle publicly and now doesn't even work in the field. This, unfortunately, is generally what happens to whistleblowers; I've seen it before with someone close. She saved lives, but now can't work in that field any more.
After you've found greener grass, if the company is large enough, write an anonymous review on Glassdoor.com calling them out for their toxic environment, so others will know to avoid that cesspool.
It's really a shame that our society shits on whistleblowers so badly.
Well, its difficult- and it's important to realise that there's not going to be some perfect resolution. A lot of people simply decide fighting isn't worth it and move on to other companies. If it's very serious, you can obviously contact the police.
In my opinion if you consider your position and you do want to pursue complaining about harassment the first thing to do is contact a third party for advice. In the UK you can contact acas.org.uk who will give impartial advice about what your rights are. I'm sure there are equivalent organisations elsewhere about what your rights are, if you really are all out of options contact /r/legaladvice but obviously take that advice with a pinch of salt.
A good rule of thumb is you need contemporaneous records of what you're complaining about - whether that is a diary or inducing the person you're accusing to put their suggestions in writing (eg, via email) and keeping a copy of that outside of work. Telling people outside of work what's happening also helps.
Obviously solving these sorts of problems within the company's HR system isn't great, but it really is the only resolution you can have beyond law enforcement, and even if you succeed in getting your harraser disciplined or removed your career will be damaged. But the point is that if you know your legal rights and have a strong record of what you're complaining about then the HR equation changes- no longer are you someone to brush off, now you're someone who poses a legitimate threat to cost the company money (and more importantly, if the HR person screws up dealing with the issue it'll be a black mark against hem, so they may well decide it's best to take your side). But remember, every single meeting, every single issue needs recording, time and date. And it's for the exact reason this article demonstrates - if you have a noted timeline saying "25th July - I complained about X, I was told "A",27th July- HR told X, 28th July - X initiated a performance conversation with me" it becomes much much easier to demonstrate retaliation - and because it's easier for you to prove, it makes it much more likely the company will want to avoid you actually demonstrate that in arbitration or court.
After you've found greener grass, if the company is large enough, write an anonymous review on Glassdoor.com calling them out for their toxic environment, so others will know to avoid that cesspool.
It's really a shame that our society shits on whistleblowers so badly.