clang-cl 9.0.1 build failures

Boost.Iterator CI fails with the following errors:

```
compile-c-c++ bin.v2\libs\iterator\test\indirect_iterator_test.test\clang-win-9.0.1\debug\x86_32\cxxstd-latest-iso\threading-multi\indirect_iterator_test.obj
In file included from libs\iterator\test\indirect_iterator_test.cpp:25:
In file included from .\boost/shared_ptr.hpp:17:
In file included from .\boost/smart_ptr/shared_ptr.hpp:28:
.\boost/smart_ptr/detail/spinlock_pool.hpp(77,5): error: no matching constructor for initialization of 'std::atomic_flag'
BOOST_DETAIL_SPINLOCK_INIT, BOOST_DETAIL_SPINLOCK_INIT, BOOST_DETAIL_SPINLOCK_INIT, BOOST_DETAIL_SPINLOCK_INIT, BOOST_DETAIL_SPINLOCK_INIT,
^~~~~~~~~~~~~~~~~~~~~~~~~~
.\boost/smart_ptr/detail/spinlock_std_atomic.hpp(88,38): note: expanded from macro 'BOOST_DETAIL_SPINLOCK_INIT'
#define BOOST_DETAIL_SPINLOCK_INIT { ATOMIC_FLAG_INIT }
^~~~~~~~~~~~~~~~
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\include\atomic(158,26): note: expanded from macro 'ATOMIC_FLAG_INIT'
#define ATOMIC_FLAG_INIT {0}
^~~
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\include\atomic(169,2): note: candidate constructor not viable: no known conversion from 'int' to 'const std::atomic_flag' for 1st argument
atomic_flag(const atomic_flag&) = delete;
^
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\include\atomic(168,2): note: candidate constructor not viable: requires 0 arguments, but 1 was provided
atomic_flag() noexcept = default;
^
```

See [here](https://ci.appveyor.com/project/Lastique/iterator/builds/53068028/job/9a969r9a51bu82ge?fullLog=true#L2002) for the full log.

I've also noticed similar failures in Boost.Log [here](https://github.com/boostorg/log/actions/runs/19117847394/job/54631341160) with clang 3.5 and 3.6, but those compilers were rather old. This one is not.