[ThinLTO] Fix ThinLTO crash

Summary:
Follow up to fix in r311023, which fixed the case where the combined
index is written to disk. The same samplePGO logic exists for the
in-memory index when computing imports, so we need to filter out
GlobalVariable summaries there too.

Reviewers: davidxl

Subscribers: inglorion, llvm-commits

Differential Revision: https://reviews.llvm.org/D36919

llvm-svn: 311254