swift234

Drop uses of Optional's variadic forwarding constructor.

In preparation for the switch to llvm::Optional, which doesn't have this.
I suggested it, but got some pushback from David Blaikie, which is
understandable because implicit conversions are dangerous.
std::experimental::optional also doesn't have this, and we don't use it
too much. The only cost is being slightly more explicit when relying on
conversions to give us a T to then wrap in an Optional.

Swift SVN r22472