Summary: Add some user facing documentation on ThinLTO and how to use it.

Reviewers: mehdi_amini

Subscribers: mehdi_amini, cfe-commits

Differential Revision: https://reviews.llvm.org/D24806

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@282089 91177308-0d34-0410-b5e6-96231b3b80d8