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

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(); }

Differential Revision: https://reviews.llvm.org/D57355

llvm-svn: 352561