IPO: Const correctness for summaries passed into passes.

Pass const qualified summaries into importers and unqualified summaries into
exporters. This lets us const-qualify the summary argument to thinBackend.

Differential Revision: https://reviews.llvm.org/D31230

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298534 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed