Fix the -print-multi-directory flag to print the selected multilib.

Summary: Fix -print-multi-directory to print the selected multilib

Reviewers: jroelofs

Reviewed By: jroelofs

Subscribers: jroelofs, timshen, thakis, srhines, cfe-commits

Differential Revision: https://reviews.llvm.org/D51354

llvm-svn: 341547