Enable auto-linking on Windows

The MSVC driver and clang do not link against the C++ runtime
explicitly.  Instead, they rely on the auto-linking via the pragma
(through `use_ansi.h`) to link against the correct version of the C++
runtime.  Attempt to do something similar here so that linking real C++
code on Windows does not require the user to explicitly specify
`c++.lib` when using libc++ as a C++ runtime on windows.

llvm-svn: 319816