Use LTO capable linker

Summary:
In DistributionExample.cmake be sure we use a LTO
capable linker, the easiest to choose is lld.

Reviewers: beanz

Reviewed By: beanz

Patch By: winksaville

Subscribers: mgorny, mehdi_amini, inglorion, dexonsmith, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62279