Create a TempFile class.

This just adds a TempFile class and replaces the use in
FileOutputBuffer with it.

The only difference for now is better error handling. Followup work includes:

- Convert other user of temporary files to it.
- Add support for automatically deleting on windows.
- Add a createUnnamed method that returns a potentially unnamed
  file. It would be actually unnamed on modern linux and have a
  unknown name on windows.

llvm-svn: 318069