[gold/ThinLTO] Invoke llvm_shutdown when exiting after ThinLTO indexing

Summary:
Instead of manually invoking PrintStatistics, simply invoke
llvm_shutdown which will take care of destroying managed statics, and as
a side effect will destroy the StatisticInfo ManagedStatic, invoking
PrintStatistics when needed.

Reviewers: fhahn

Subscribers: inglorion, llvm-commits

Differential Revision: https://reviews.llvm.org/D45820

llvm-svn: 330341