[PECOFF] Properly close a file descriptor.

This was basically benign resource leak on Unix, but on Windows
it could cause builds to fail because opened file descriptor
prevents other processes from moving or removing the file.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@222799 91177308-0d34-0410-b5e6-96231b3b80d8