[ThinLTO] No need to rediscover imports in distributed backend

Summary:
We can simply import all external values with summaries included in
the individual index file created for the distributed backend job,
as only those are added to the individual index file created by the
WriteIndexesThinBackend (in addition to summaries for the original
module, which are skipped here).

While computing the cross module imports on this index would come to
the same conclusion as the original thin link import logic, it is
unnecessary work. And when tuning, it avoids the need to pass the
same function importing parameters (e.g. -import-instr-limit) to
both the thin link and the backends (otherwise they won't make the
same decisions).

Reviewers: mehdi_amini, pcc

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D28139

llvm-svn: 290674