Such as the one in the darwin-dsymutil.c test.
The darwin-dsymutil.c tests this code path right now on line 33. I found this error by running it under asan; otherwise it was fully latent.
It seems pretty clear to me that when End == -1 (StringRef::npos), creating a string ref from that is obviously a bug.
I'm not even sure how to check for a failure. clang will almost never crash; there is just a very subtle bounds violation or empty stringref. How it manifests itself depends on subsequent usage of the stringref, and stack layout.