Open fstream files in O_CLOEXEC mode when possible.

Reviewers: EricWF, mclow.lists, ldionne

Reviewed By: ldionne

Subscribers: smeenai, dexonsmith, christof, ldionne, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D59839

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@372027 91177308-0d34-0410-b5e6-96231b3b80d8