[libcxx] Fix building on Windows after 346ef5e5879e9143fe008c35c3dd6ea3c402634a

[mstorsjo]

Summary

That commit, while intended to be NFC, wasn't.

libcxx/src/random.cpp used to first include <__config>, which used
to define _LIBCPP_USING_WIN32_RANDOM (but which doesn't include
any other system headers). After that, it checked for
_LIBCPP_USING_WIN32_RANDOM, and conditionally defined _CRT_RAND_S.
This was defined before including any further headers, as this has
to be defined for the declaration of rand_s() to be visible, and
has to be defined before including stdlib.h.

After that commit, _LIBCPP_USING_WIN32_RANDOM was defined only
later, in __random/random_device.h (which is included in
<random>). But we can't define _CRT_RAND_S after that, because
stdlib.h probably has been pulled in already at that point.

Instead loosen the criteria for defining _CRT_RAND_S to
_LIBCPP_WIN32API, which practically is defined exactly when
_LIBCPP_USING_WIN32_RANDOM is defined.

[mstorsjo]

Fixed differently by rGbeff71520b5be766ef06a0e0c1e67629fa39f429.

libcxx/src/random.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include <__config>
 
-#if defined(_LIBCPP_USING_WIN32_RANDOM)
+#if defined(_LIBCPP_WIN32API)
     // Must be defined before including stdlib.h to enable rand_s().
 #   define _CRT_RAND_S
 #endif // defined(_LIBCPP_USING_WIN32_RANDOM)
 
 #include "limits"
 #include "random"
 #include "system_error"