[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178093 91177308-0d34-0410-b5e6-96231b3b80d8
[analyzer] Better test for r178063.

Jordan pointed out that my previously committed test was bogus.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178094 91177308-0d34-0410-b5e6-96231b3b80d8