[lto] Add saving the LTO .o file to -save-temps.

Summary:
This implements another part of -save-temps.
After this, the only remaining part is dumping the optimized bitcode. But
currently LLD's LTO doesn't have a non-intrusive place to put this.
Eventually we probably will and it will make sense to add it then.

Reviewers: ruiu, rafael

Subscribers: joker.eph, Bigcheese, llvm-commits

Differential Revision: http://reviews.llvm.org/D18009

llvm-svn: 263070