[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.
[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 .



git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@280058 91177308-0d34-0410-b5e6-96231b3b80d8

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