[ThinLTO] Ignore object files with no ThinLTO modules if -fthinlto-index= is set

Summary:
ThinLTO compilation may decide not to split module and keep at as regular LTO.
In this can this module already processed during indexing and already a part of
merged object file. So here we can just skip it.

Reviewers: pcc, tejohnson

Reviewed By: tejohnson

Subscribers: mehdi_amini, inglorion, eraman, cfe-commits

Differential Revision: https://reviews.llvm.org/D42680

llvm-svn: 325410