[analyzer] Teach ObjCDeallocChecker about XCTestCase

Like with SenTestCase, subclasses of XCTestCase follow a "tear down" idiom to
release instance variables and so typically do not release ivars in -dealloc.
This commit applies the existing special casing for SenTestCase to XCTestCase
as well.

rdar://problem/25884696

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@273441 91177308-0d34-0410-b5e6-96231b3b80d8