[Bug libstdc++/91067] [9/10 Regression] Clang compiler can't link executable if std::filesystem::directory_iterator is encountered

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91067

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I didn't export that because nothing should need it. Nothing in libstdc++
derives from __shared_ptr<_Dir> and nothing in user code is allowed to refer to
that type, because it's an implementation detail.

The explicit instantiation tells Clang not to emit that constructor, but it
shouldn't cause it to depend on that constructor.