Revert: [llvm] r325448 - [ThinLTO] Add GraphTraits for FunctionSummaries

Add GraphTraits definitions to the FunctionSummary and ModuleSummaryIndex classes. These GraphTraits will be used to construct find SCC's in ThinLTO analysis passes.

Second attempt, since last patch caused stage2 build to fail (now using function_ref rather than std::function).

Reverted due to buildbot failures

llvm-svn: 325454