[libc++] Fix Windows build error in <functional>

[thomasanderson]

Summary

On my Windows system, __allocator is defined to nothing. This change fixes build errors of the below form:

In file included from algorithm:644:
functional(1492,31):  error: expected member name or ';' after declaration specifiers
    const _Alloc& __allocator() const { return __f_.second(); }

[EricWF]

Please add __allocator to nasty_macros.hpp (https://github.com/llvm-mirror/libcxx/blob/master/test/support/nasty_macros.hpp#L57)

Otherwise, this LGTM.

include/functional

class __alloc_func<_Fp, _Ap, _Rp(_ArgTypes...)>
 
   public:
     typedef _Fp _Target;
     typedef _Ap _Alloc;
 
     _LIBCPP_INLINE_VISIBILITY
     const _Target& __target() const { return __f_.first(); }
 
+    // MSVC may define __allocator, so use __get_allocator instead.
     _LIBCPP_INLINE_VISIBILITY
-    const _Alloc& __allocator() const { return __f_.second(); }
+    const _Alloc& __get_allocator() const { return __f_.second(); }
 
     _LIBCPP_INLINE_VISIBILITY
     explicit __alloc_func(_Target&& __f)
         : __f_(piecewise_construct, _VSTD::forward_as_tuple(_VSTD::move(__f)),
                _VSTD::forward_as_tuple())
     {
     }
 

 };
 
 template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
 __base<_Rp(_ArgTypes...)>*
 __func<_Fp, _Alloc, _Rp(_ArgTypes...)>::__clone() const
 {
     typedef allocator_traits<_Alloc> __alloc_traits;
     typedef typename __rebind_alloc_helper<__alloc_traits, __func>::type _Ap;
-    _Ap __a(__f_.__allocator());
+    _Ap __a(__f_.__get_allocator());
     typedef __allocator_destructor<_Ap> _Dp;
     unique_ptr<__func, _Dp> __hold(__a.allocate(1), _Dp(__a, 1));
     ::new ((void*)__hold.get()) __func(__f_.__target(), _Alloc(__a));
     return __hold.release();
 }
 
 template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
 void
 __func<_Fp, _Alloc, _Rp(_ArgTypes...)>::__clone(__base<_Rp(_ArgTypes...)>* __p) const
 {
-    ::new (__p) __func(__f_.__target(), __f_.__allocator());
+    ::new (__p) __func(__f_.__target(), __f_.__get_allocator());
 }
 
 template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
 void
 __func<_Fp, _Alloc, _Rp(_ArgTypes...)>::destroy() _NOEXCEPT
 {
     __f_.destroy();
 }
 
 template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
 void
 __func<_Fp, _Alloc, _Rp(_ArgTypes...)>::destroy_deallocate() _NOEXCEPT
 {
     typedef allocator_traits<_Alloc> __alloc_traits;
     typedef typename __rebind_alloc_helper<__alloc_traits, __func>::type _Ap;
-    _Ap __a(__f_.__allocator());
+    _Ap __a(__f_.__get_allocator());
     __f_.destroy();
     __a.deallocate(this, 1);
 }
 
 template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
 _Rp
 __func<_Fp, _Alloc, _Rp(_ArgTypes...)>::operator()(_ArgTypes&& ... __arg)
 {
private:
     }
 
     template <typename _Fun> static void __large_destroy(void* __s)
     {
         typedef allocator_traits<typename _Fun::_Alloc> __alloc_traits;
         typedef typename __rebind_alloc_helper<__alloc_traits, _Fun>::type
             _FunAlloc;
         _Fun* __f = static_cast<_Fun*>(__s);
-        _FunAlloc __a(__f->__allocator());
+        _FunAlloc __a(__f->__get_allocator());
         __f->destroy();
         __a.deallocate(__f, 1);
     }
 
     template <typename _Fun>
     _LIBCPP_INLINE_VISIBILITY static const __policy*
         __choose_policy(/* is_small = */ false_type)
     {