This array-like wrapper adapts `MDTuple` to have elements of `MDTypeRef`
(whereas `MDTypeArray` has elements of `MDType`). This is necessary to
migrate code using `DITypeArray`. The only use of this is
`MDSubroutineType`'s `getTypeArray()` accessor.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234200 91177308-0d34-0410-b5e6-96231b3b80d8