[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.

llvm-svn: 159558