[libc++] Roll up fstream support into filesystem support

LIBCXX_ENABLE_FILESYSTEM should represent whether the platform has
support for a filesystem, not just whether we support <filesystem>.
This patch slightly generalizes the setting to also encompass whether
we provide <fstream>, since that only makes sense when a filesystem is
supported.

Differential Revision: https://reviews.llvm.org/D152168

GitOrigin-RevId: 66a562d22e708ba40b8443b58e504ac3f983ba59