Use SafelyCloseFileDescriptor instead of close.

Opening a file using openFileForWrite and closing it using close
was asymmetric. It also had a subtle portability problem (details are
described in the commit message for r219189).

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@222802 91177308-0d34-0410-b5e6-96231b3b80d8