[analyzer] Weaken assertion in trackNullOrUndefValue()

We should ignore paren casts when making sure that the semantic expression
in a PseudoObjectExpr for an ObjC getter is a message send.

This has no other intended functionality change.

Adding a test for this exposed an interesting issue in another test case
that only manifests under ARC. trackNullOrUndefValue() is not properly
suppressing for nil values that are the result of nil propagation from a nil
receiver when the nil is returned from a function. I've added a FIXME for that
missing suppression.

rdar://problem/27290568

llvm-svn: 279181