[libc++] Re-document how to use <filesystem> with various versions of libc++

This documentation was removed when we added <filesystem> to the dylib
in r356518, but it really should have been updated to reflect the new
state of things. Keeping documentation around doesn't hurt and users
will have an easier time migrating.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@356681 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed