Abseil types are untested for exception-safety

Google internally doesn't use exceptions in our C++ code. Thus, our claim that Abseil types are battle-tested comes with an asterisk that they haven't been so thoroughly tested in the face of exceptions.

We are working internally on a framework to thoroughly test the exception-safety of our types which care about it, and as it is finished we will begin rolling out exception-safety bugfixes. We are currently aiming for basic exception safety everywhere, and strong exception safety only where reasonable and where it won't affect performance.

This Issue is a catch-all for reports of exception-unsafety of Abseil types.