[fs.dir.entry.obs] directory_entry members to call non-existent functions

[BoPersson]

The description for directory_entry::exists(error_code& ec) requires it to call

(std::filesystem::) exists(this->status(), ec)

but there is no such function. Probably intended to be a call to filesystem::exists(this->status(ec));

Most member functions in this subclause seem to have the same issue.

[jwakely]

Introduced by 221d52e but http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0317r1.html is correct, so it's an editorial mistake applying the paper.