[Driver] Support for -save-stats in AddGoldPlugin.

This patch updates AddGoldPlugin to pass stats-file to the Gold plugin,
if -save-stats is passed. It also moves the save-stats option handling
to a helper function tools::getStatsFileName.

Reviewers: tejohnson, mehdi_amini, compnerd

Reviewed By: tejohnson, compnerd

Differential Revision: https://reviews.llvm.org/D45771

llvm-svn: 330422