ARCMigrator is using code from RetainCountChecker, which is a layering violation (and it also does it badly, by using a different header, and then relying on implementation being present in a header file).
This change splits up RetainSummaryManager into a separate library in lib/Analysis, which can be used independently of a checker.
The tradition here seems to be to only place _ where there should be /, i.e. LLVM_CLANG_LIB_ANALYSIS_RETAINSUMMARYMANAGER.