[libcxx] Add a missing include for __bit_iterator

This is to fix compile error with explicit Clang modules like
```
../../third_party/libc++/src/include/__vector/vector_bool.h:85:11: error: default argument of '__bit_iterator' must be imported from module 'std.bit_reference_fwd' before it is required
   85 |   typedef __bit_iterator<vector, false> pointer;
      |           ^
../../third_party/libc++/src/include/__fwd/bit_reference.h:23:68: note: default argument declared here is not reachable
   23 | template <class _Cp, bool _IsConst, typename _Cp::__storage_type = 0>
      |                                                                    ^
```

Author: atetubou

libcxx/include/__vector/vector_bool.h

@@ -18,6 +18,7 @@
 #include <__bit_reference>
 #include <__config>
 #include <__functional/unary_function.h>
+#include <__fwd/bit_reference.h>
 #include <__fwd/functional.h>
 #include <__fwd/vector.h>
 #include <__iterator/distance.h>