When building with clang-cl targeting MSVC, the __int128_t type is available, but the runtime helpers (normally provided by libgcc or compiler_rt builtins) aren't available; bundle them within libc++ in those cases. (The same is already done for one function that is missing in libgcc but present in compiler_rt.)
This is an alternative to D91139 (which just disables the use of int128_t for MSVC targets). Either of these has to be picked before settling down the ABI of libc++ filesystem on MSVC targets. This one on the other hand hardcodes the clang-specific int128_t into the ABI for MSVC targets.
(For MinGW targets, none of this is an issue as libgcc and/or compiler_rt are available.)