[PATCH] Avoid allocator operator== in _Safe_container

[François Dumont]

Some newly introduced tests in 
23_containers/unordered_map/cons/default.cc revealed that we are forcing 
the allocator type to have a operator==.

It can be avoided by checking allocator_traits::is_always_equal.

     libstdc++: [_GLIBCXX_DEBUG] Avoid allocator operator== when always 
equal

     Use std::allocator_traits::is_always_equal to find out if we need 
to compare
     allocator instances on safe container allocator aware move constructor.

     libstdc++-v3/ChangeLog:

             * include/debug/safe_container.h
             (_Safe_container(_Safe_container&&, const _Alloc&, 
std::true_type)): New.
             (_Safe_container(_Safe_container&&, const _Alloc&, 
std::false_type)): New.
             (_Safe_container(_Safe_container&&, const _Alloc&)): Use 
latters.

Tested under Linux x86 Debug mode.

Ok to commit ?

François

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