Add missing `nullptr` from `MDSubroutineType`'s operands for
`MDCompositeTypeBase::getIdentifier()` (and add tests for all the other
unused fields). This highlights just how crazy it is that
`MDSubroutineType` inherits from `MDCompositeTypeBase`.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229926 91177308-0d34-0410-b5e6-96231b3b80d8