[libc++] Potential bugprone-use-after-move in std::map

[mordante]

See review https://reviews.llvm.org/D67086

 template <class _Key, class _Tp, class _Compare, class _Allocator>
 _Tp&
 map<_Key, _Tp, _Compare, _Allocator>::operator[](key_type&& __k)
 {  
+     // TODO investigate this clang-tidy warning.
+     // NOLINTNEXTLINE(bugprone-use-after-move)
      return __tree_.__emplace_unique_key_args(__k,
          _VSTD::piecewise_construct,
          _VSTD::forward_as_tuple(_VSTD::move(__k)),
          _VSTD::forward_as_tuple()).first->__get_value().second;
}

As @ldionne mentioned "Can you file a Github issue and link it here? In the issue you can say something like "we have a potential use-after-move in map::operator[]" and link to https://godbolt.org/z/e1x9c1h6Y."