MacOSKeychainAPIChecker: There is no need to use SymbolMetadata to represent the allocated data symbol, we can just use the symbol corresponding to the SymbolicRegion. This simplifies tracking of the symbol, for example, SymbolMetadata needs to go through extra hoops to stay alive.

Make AllocationState internal to the MacOSKeychainAPIChecker class.

llvm-svn: 137514