[libcxx] [test] limited_allocator should be limited_allocator's best friend forever

[STL_MSFT]

Summary

limited_allocator must grant friendship in order to be rebindable. Found by MSVC, which rebinds allocators in more situations than libc++ does.

This raises an interesting question as to whether libc++ should be validating user-defined allocators more aggressively.

[EricWF]

I committed this fix a couple days ago when helping the libstdc++ maintainers setup our test suite.

test/support/test_allocator.h

struct limited_alloc_handle {
   }
 };
 
 template <class T, std::size_t N>
 class limited_allocator
 {
     typedef limited_alloc_handle<N> BuffT;
     std::shared_ptr<BuffT> handle_;
+
+    template <class U, std::size_t X> friend class limited_allocator;
+
 public:
     typedef T                 value_type;
     typedef value_type*       pointer;
     typedef const value_type* const_pointer;
     typedef value_type&       reference;
     typedef const value_type& const_reference;
     typedef std::size_t       size_type;
     typedef std::ptrdiff_t    difference_type;