Add the -disable-opt option to LTO. This adds:

- Consistency with opt (which supports the same option with the same meaning and
  description).
- Debugging gold plugin-based linking without optimizations getting in the way.
- Debugging programs linked with the gold plugin while preserving the original
  debug info.
- Fine-grained control over LTO passes using the gold plugin in combination with
  opt (or clang/dragonegg).

Patch by Cristiano Giuffrida!