[analyzer] Make sure IDC works for ‘NSContainer value/key is nil’ checks.

Register the nil tracking visitors with the region and refactor trackNullOrUndefValue a bit.

Also adds the cast and paren stripping before checking if the value is an OpaqueValueExpr
or ExprWithCleanups.
[analyzer] Better test for r178063.

Jordan pointed out that my previously committed test was bogus.