On Windows, delete existing log file before renaming temp file.
On Windows, delete existing log file before renaming temp file.

On non-Windows platforms, os.rename() will silently replace the
destination file if it already exists.  On Windows, it doesn't do
this, and the filesystem has no mechanism to simulate the same type
of atomic rename operation.  So on Windows, delete the file first
before calling os.rename().

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@238239 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb