Avoid leaking log file descriptors into the inferior process.

Summary:
This commit adds a new open flag File::eOpenOptionCloseOnExec (i.e., O_CLOEXEC), and adds it to
the list of flags when opening log files (#ifndef windows). A regression test is included.

Reviewers: vharron, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D7412

llvm-svn: 228310