[libc++] Fix incorrect availability markup for bad_optional_access & friends

In 7fb40e15, I changed the availability for bad_optional_access and
friends from macOS 10.14 to 10.13 after conducting an investigation on
old dylibs. It turns out that macOS 10.13 did have bad_optional_access,
however the dylib on iOS didn't match the dylib on macOS, so those
exception classes were only introduced in iOS 12.

Thanks to Aditya Kumar for noticing this.

Differential Revision: https://reviews.llvm.org/D121735