Move MSVC workarounds for future<Error>/future<Expected<T>> out of ORC and into

a header in support.

MSVC's std::future implementation requires types to be default constructible,
but Error and Expected are not. This issue came up once before in ORC's
RPCUtils.h header and was worked around there but came up again in r342939, so
I am moving the workaround to Support to make it available to other clients.

llvm-svn: 343011