[analyzer] Move the last bits of CallOrObjCMessage over to CallEvent.

This involved refactoring some common pointer-escapes code onto CallEvent,
then having MallocChecker use those callbacks for whether or not to consider
a pointer's /ownership/ as escaping. This still needs to be pinned down, and
probably we want to make the new argumentsMayEscape() function a little more
discerning (content invalidation vs. ownership/metadata invalidation), but
this is a good improvement.

As a bonus, also remove CallOrObjCMessage from the source completely.

llvm-svn: 159557