I believe I have found and updated all the lit tests that depended on the triple-substring feature, and I’ve committed D139869 as mentioned in the previous comment. There is one remaining review for an update to some Polly tests, D139728, however that update corrects misuse of triple components in the REQUIRES clause, and therefore those tests never ran. I don’t view that as a blocker.
The remaining steps are to post something in the Announcements category, and actually remove the triple-substring feature from lit. That removal will catch any places that I missed during my passes over the project’s lit tests 
Maybe worth putting something in the Release Notes as well? This change will affect downstream tests that depend on the feature, as well as non-LLVM users of lit (I’m told there may be some).
I’m inclined to do the Announcements post today or tomorrow, but put off the final removal of the lit feature until the new year. I’m on holiday starting the 23rd myself, so I won’t be around to deal with any fallout that doesn’t get reported right away.