[PATCH] Add an emplace(...) method to llvm::Optional<T>.

[Jordan Rose]

We're using it internally in our own version of Optional; my guess would be that it would only be for projects that had stricter requirements than LLVM and Clang do (like lld, which requires MSVS 2012). However, I could add the fake-variadic up-to-six-arguments fallback for now.

http://reviews.llvm.org/D5508