As explained in __config, we have an ABI tag that we use to ensure that
we don't run into ODR issues when mixing different versions of libc++
in multiple TUs. However, the reasoning behind that extends not only to
different versions of libc++, but also to different configurations of
the same version of libc++. In fact, we've been aware of this for a while
but never really bothered to make the change because ODR issues are often
thought to be benign.

Well, it turns out that I just spent over an hour banging my head against
an issue that boils down to our lack of encoding some ODR properties in
the ABI tag, so here's the patch we should have done a long time ago.

For now, the ODR properties we encode in the ABI tag are:
- library version
- exceptions vs no-exceptions
- hardening mode

Those are all things that we support different values for on a per-TU
basis and they definitely affect ODR in a meaningful way. We can add
more properties later as we see fit.