[libc++] Expand the contents of LIBCXX_ENABLE_FILESYSTEM

Since LIBCXX_ENABLE_FILESYSTEM now truly represents whether the
platform supports a filesystem (as opposed to whether the <filesystem>
library is provided), we can provide a few additional classes from
the <filesystem> library even when the platform does not have support
for a filesystem. For example, this allows performing path manipulations
using std::filesystem::path even on platforms where there is no actual
filesystem.

rdar://107061236

Differential Revision: https://reviews.llvm.org/D152382

GitOrigin-RevId: c352fa7407122ee62d990d6b82551650149f98d4
152 files changed