This patch continues the work of moving codes out of CFLSteensAliasAnalysis so that CFLAndersAliasAnalysis could also use them.
StratifiedAttrs is one such utility. It has been renamed to AliasAttrs, since its usage may no longer be local to StratifiedSets any more.
Summary-related structs, such as InterfaceValue, ExternalRelation, etc, are also refactored out. Again, the idea is to leave only Steensgard-specific codes in CFLSteensAliasAnalysis.cpp, and put summary-related codes in AliasAnalysisSummary.cpp