[ThinLTO] Include linkage type in function summary

[ThinLTO] Include linkage type in function summary

Summary:
Adds the linkage type to both the per-module and combined function
summaries, which subsumes the current islocal bit. This will eventually
be used to optimized linkage types based on global summary-based
analysis.

Reviewers: joker.eph

Subscribers: joker.eph, davidxl, llvm-commits

Differential Revision: http://reviews.llvm.org/D16943

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@259993 91177308-0d34-0410-b5e6-96231b3b80d8