Fix size_t/off_t mixup in std::filesystem.

Summary: ftruncate takes an off_t, not a size_t.

Reviewers: EricWF, mclow.lists

Reviewed By: EricWF

Subscribers: christof, ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D56578

llvm-svn: 351226