[analyzer] Fixup for r155244: PurgeDeadSymbols should subclass StmtPoint

To solve the inconsistency pointed out in Erik's review, refactor class
hierarchy of ProgramPoints so that PreStmtPurgeDeadSymbols and
PostStmtPurgeDeadSymbols both subclass from StmtPoint instead of
PostStmt.

llvm-svn: 156315