[libc++] Mark internal types of std::filesystem as hidden

[ldionne]

Summary

Otherwise, implicit instantiations of templates with these types can
cause the dylib to start exporting the vtable/RTTI of the instantiation.
Giving hidden visibility to those types causes the compiler to understand
that they are not used outside the dylib, and as a result implicitly
instantiated vtables/RTTI of templates with those internal types will
get hidden visibility.

libcxx/trunk/include/filesystem

public:
   _LIBCPP_INLINE_VISIBILITY
   const char* what() const noexcept override {
     return __storage_->__what_.c_str();
   }
 
   void __create_what(int __num_paths);
 
 private:
-  struct _Storage {
+  struct _LIBCPP_HIDDEN _Storage {
     _LIBCPP_INLINE_VISIBILITY
     _Storage(const path& __p1, const path& __p2) : __p1_(__p1), __p2_(__p2) {}
 
     path __p1_;
     path __p2_;
     string __what_;
   };
   shared_ptr<_Storage> __storage_;

 
 inline _LIBCPP_INLINE_VISIBILITY path weakly_canonical(path const& __p,
                                                        error_code& __ec) {
   return __weakly_canonical(__p, &__ec);
 }
 
 class directory_iterator;
 class recursive_directory_iterator;
-class __dir_stream;
+class _LIBCPP_HIDDEN __dir_stream;
 
 class directory_entry {
   typedef _VSTD_FS::path _Path;
 
 public:
   // constructors and destructors
   directory_entry() noexcept = default;
   directory_entry(directory_entry const&) = default;
private:
 
   _LIBCPP_FUNC_VIS
   void __pop(error_code* __ec = nullptr);
 
   inline _LIBCPP_INLINE_VISIBILITY friend bool
   operator==(const recursive_directory_iterator&,
              const recursive_directory_iterator&) noexcept;
 
-  struct __shared_imp;
+  struct _LIBCPP_HIDDEN __shared_imp;
   shared_ptr<__shared_imp> __imp_;
   bool __rec_;
 }; // class recursive_directory_iterator
 
 inline _LIBCPP_INLINE_VISIBILITY bool
 operator==(const recursive_directory_iterator& __lhs,
            const recursive_directory_iterator& __rhs) noexcept {
   return __lhs.__imp_ == __rhs.__imp_;