raw_fd_ostream: Make file handles non-inheritable by default

Summary:
This makes the file descriptors on unix platform non-inheritable (O_CLOEXEC).

There is no change in behavior on windows, as the handles were already
non-inheritable there.

Reviewers: rnk, rafael

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D28854

llvm-svn: 292401