clean up use of _WIN32

Replace the use of _WIN32 in libc++. Replace most use with a C runtime
check _LIBCPP_MSVCRT or the new _LIBCPP_WIN32 to indicate that we are
using the Win32 API. Use a new _LIBCPP_WCHAR_IS_UCS2 to indicate that we
are on an environment that has a short wchar_t.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290910 91177308-0d34-0410-b5e6-96231b3b80d8

src/new.cpp

@@ -72,7 +72,7 @@ operator new(std::size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC
     if (static_cast<size_t>(alignment) < sizeof(void*))
       alignment = std::align_val_t(sizeof(void*));
     void* p;
-#if defined(_WIN32)
+#if defined(_LIBCPP_MSVCRT)
     while ((p = _aligned_malloc(size, static_cast<size_t>(alignment))) == nullptr)
 #else
     while (::posix_memalign(&p, static_cast<size_t>(alignment), size) != 0)