Summary:
When creating a FileSP object, do not flush() the underlying file unless
it is open for writing. Attempting to flush() a read-only fd results
in EBADF on NetBSD.
Reviewers: lawrence_danna, labath, krytarowski
Reviewed By: lawrence_danna, labath
Subscribers: lldb-commits
Differential Revision: https://reviews.llvm.org/D69320