tl::optional cannot be used with exceptions disabled

[FrancisRussell]

It's not possible to use tl::optional with exceptions disabled. It would be useful for code that is required to compile without exception support if this were possible. With libstdc++, abort is called instead for conditions that would raise an exception.

It unfortunately varies across compilers how to detect if exception support has been disabled, but in a third party variant implementation this is done by https://github.com/mpark/variant/blob/d1cdfdd3f2ed80710ba4d671fe6bffaa3e28201a/v1.4.0/variant.hpp#L270 with __has_feature defined for non-clang compilers as https://github.com/mpark/variant/blob/d1cdfdd3f2ed80710ba4d671fe6bffaa3e28201a/v1.4.0/variant.hpp#L232.