ST->scanReachableSymbols() is creating a SubRegionMap (SRM) on every call...

ST->scanReachableSymbols() is creating a SubRegionMap (SRM) on every call since one SRM is created in each ScanReachableSymbols instance. Creating the object just once and calling only scan inside the loop gives ~ 14% speed up of the StaticAnalyzer run (Release+Asserts). 

Pull out the declaration of the ScanReachableSymbols so that it can be used directly. Document ProgramState::scanReachableSymbols() methods.

llvm-svn: 140323