[Support] Add a conditionally defined default constructor (available on MSVC

only) for Expected<T> so that it can interoperate with MSVC's std::future
implementation.

MSVC 2013's std::future implementation requires the wrapped type to be default
constructible.

Hopefully this will fix the bot breakage in
http://lab.llvm.org:8011/builders/clang-x86-win2008-selfhost/builds/9937 .

llvm-svn: 280058