[analyzer] Add a new abstraction over all types of calls: CallEvent

This is intended to replace CallOrObjCMessage, and is eventually intended to be
used for anything that cares more about /what/ is being called than /how/ it's
being called. For example, inlining destructors should be the same as inlining
blocks, and checking __attribute__((nonnull)) should apply to the allocator
calls generated by operator new.
[analyzer] Convert RetainCountChecker to use CallEvent as much as possible.

This ended allowing quite a bit of cleanup, and some minor changes.

- CallEvent makes it easy to use hasNonZeroCallbackArg more aggressively, which
  we check in order to avoid false positives with callbacks that might release
  the object.
- In order to support this for functions which consume their arguments, there
  are two new ArgEffects: DecRefAndStopTracking and DecRefMsgAndStopTracking.
  These act just like StopTracking, except that if the object only had a
  return count of +1 it's now considered released instead (so we still get
  use-after-free messages).
- On the plus side, we no longer have to special-case
  +[NSObject performSelector:withObject:afterDelay:] and friends.
- The use of IdentifierInfos in the method summary cache is now hidden; only
  the ObjCInterfaceDecl gets passed around most of the time.
- Since we cache all "simple" summaries and check every function call, there is
  no real benefit to having NULL stand in for default summaries anymore.
- Whitespace, unused methods, etc.

Even more simplification to come when we get check::postCall and can unify all
these other post* checks.
[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>.
[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.
[analyzer] Begin replacing ObjCMessage with ObjCMethodCall and friends.

Previously, the CallEvent subclass ObjCMessageInvocation was just a wrapper
around the existing ObjCMessage abstraction (over message sends and property
accesses). Now, we have abstract CallEvent ObjCMethodCall with subclasses
ObjCMessageSend and ObjCPropertyAccess.

In addition to removing yet another wrapper object, this should make it easy
to add a ObjCSubscriptAccess call event soon.
[analyzer] Finish replacing ObjCMessage with ObjCMethodDecl and friends.

The preObjCMessage and postObjCMessage callbacks now take an ObjCMethodCall
argument, which can represent an explicit message send (ObjCMessageSend) or an
implicit message generated by a property access (ObjCPropertyAccess).
[analyzer] Use CallEvent for inlining and call default-evaluation.
[analyzer] Convert CXXConstructExpr over to use CallEvent for evaluation.
[analyzer] Add generic preCall and postCall checks.
[analyzer] Convert existing checkers to use check::preCall and check::postCall.