[analyzer] Convert CallAndMessageChecker and ObjCSelfInitChecker to CallEvent.

Both of these got uglier rather than cleaner because we don't have preCall and
postCall yet; properly wrapping a CallExpr in a CallEvent requires doing a bit
of deconstruction on the callee. Even when we have preCall and postCall we may
want to expose the current CallEvent to pre/postStmt<CallExpr>.

llvm-svn: 159556