`<optional>`: address sanitizer annotation

[AlexGuteniev]

We can annotate empty optional.

This may help finding dangling references to objects in optionals that have been reset to empty state.

[StephanTLavavej]

Agreed that this is worth doing. optional::operator* was surprisingly-to-me one of the heavy hitters for security issues, addressed by STL Hardening. Having additional ASan defenses would be great.

I am somewhat confused by the machinery around string and vector's ASan annotations, but at least we have precedent there.