[libc++] Disable int128_t and ship filesystem on MSVC by default

Back in 2020 [1], we went very close to enabling Filesystem on MSVC
by disabling int128_t, but decided to wait because MSVC support
for int128_t was supposed to come shortly after. Since it's not
there yet, I propose turning off int128_t support by default on MSVC.
This will make <filesystem> available by default on MSVC, and most
importantly will open the possibility for changing
LIBCXX_ENABLE_FILESYSTEM to mean "the system doesn't have support
for a filesystem" instead of simply "don't build the std::filesystem
library", which is what I'm really after with this change.

In a way, this is a resurection of D91139.

[1]: https://reviews.llvm.org/D91139#2429595

Differential Revision: https://reviews.llvm.org/D134912