[analyzer] Combine all ObjC message CallEvents into ObjCMethodCall.

As pointed out by Anna, we only differentiate between explicit message sends

This also adds support for ObjCSubscriptExprs, which are basically the same
as properties in many ways. We were already checking these, but not emitting
nice messages for them.

This depends on the llvm::PointerIntPair change in r160456.

llvm-svn: 160461