Revert "[ThinLTO] Add GraphTraits for FunctionSummaries"

It caused assertion failure
Assertion failed: (!DD.IsLambda && !MergeDD.IsLambda && "faked up lambda definition?"), function MergeDefinitionData, file /Users/buildslave/jenkins/workspace/clang-stage1-configure-RA/llvm/tools/clang/lib/Serialization/ASTReaderDecl.cpp, line 1675.

on the second stage build bots.

llvm-svn: 324932