<ext/bitmap_allocator.h> includes <function>, and since C++17 that
includes <unordered_map>. If std::allocator is defined in terms of
__gnu_cxx::bitmap_allocator then you get a circular reference and
bootstrap fails when compiling src/c++17/*.cc.
libstdc++-v3/ChangeLog:
PR libstdc++/103381
* include/ext/bitmap_allocator.h: Include <bits/stl_function.h>
instead of <functional>.
