Allow disabling of filesystem library.

Summary: Filesystem doesn't work on Windows, so we need a mechanism to turn it off for the time being.

Reviewers: ldionne, serge-sans-paille, EricWF

Reviewed By: EricWF

Subscribers: mstorsjo, mgorny, christof, jdoerfert, libcxx-commits

Differential Revision: https://reviews.llvm.org/D59619

llvm-svn: 356633
GitOrigin-RevId: f1d87f8b4c430d72e7ab07e13508336a7841c3e6
8 files changed