-fsanitize=undefined -ftrapv flags not supported

[dcleblanc]

The code is untested with the following g++ or clang++ flags:
-fsanitize=undefined -ftrapv

It might crash if there was an oversight. The work item that's needed is to add these flags to the test harness, and fix anything that results.

[dcleblanc]

This has now been added to the clang test harness, and observed to now run cleanly

Thanks to practicalswift@gmail.com for bringing this to my attention.