Also add the corresponding XFAILs to tests that require filesystem.
Note that I did try to use a less intrusive approach using pragma
clang attribute, but this didn't work due to apparent bugs in Clang
(which should be fixed independently but should not delay this change).
Also note that we mark the functions as being introduced in versions
99.99 of the platforms because of a Clang bug (PR40991), which causes
a hard error when an unavailable function is used to initialize the
default argument of another unavailable function. We run into this
issue with functions like proximate().