[ThinLTO] PrintStatistics when we exit early for thinlto-index-only

Summary:
This is necessary to get stats from the ThinLink printed before the
early exit when compiling in a distributed build.

Reviewers: mehdi_amini

Subscribers: Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D29461

llvm-svn: 293908