[LTO] Add stats-file option to LTO/Config.h.

This patch adds a StatsFile option to LTO/Config.h and updates both
LLVMGold and llvm-lto2 to set it.

Reviewers: MatzeB, tejohnson, espindola

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D45531

llvm-svn: 330411