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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@319816 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

@@ -1265,6 +1265,13 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
 # endif
 #endif // defined(_LIBCPP_HAS_NO_PRAGMA_PUSH_POP_MACRO)
 
+#if defined(_LIBCPP_ABI_MICROSOFT) && !defined(_LIBCPP_BUILDING_LIBRARY)
+# if defined(_DLL)
+#   pragma(lib, "c++.lib")
+# else
+#   pragma(lib, "libc++.lib")
+# endif
+#endif // defined(_LIBCPP_ABI_MICROSOFT) && !defined(_LIBCPP_BUILDING_LIBRARY)
 
 #endif // __cplusplus