rtc::Optional<T>: Don't secretly contain a default-constructed T when empty

Instead, use a neat trick with union to ensure that we have a T only
when we're supposed to (and just a bunch of unused memory otherwise).
This is how std::optional behaves, so it makes sense for us to do the
same (and it's convenient, too, since we don't have to pay for the
default-constructed T, and we support types that don't have default
constructors).

Doing this became possible recently when we dropped support for MSVC
2013, which didn't support unions containing non-trivial types.

Review-Url: https://codereview.webrtc.org/1896833004
Cr-Commit-Position: refs/heads/master@{#12664}