[PATCH] Add a forwarding variadic constructor to llvm::Optional<T>.

[Jordan Rose]

Another interesting case that came up was passing an value of type Foo * to a parameter of type Optional<FooWrapper>.

I did forget to include the implicitly-constructible-only constraint, but I think I'm going to just abandon this; we weren't using it quite as frequently as I remembered, and it's not something the standard will ever do.

http://reviews.llvm.org/D5511