Remove the accessors of `DIDerivedType` that downcast to
`MDDerivedType`, shifting the `cast<MDDerivedType>` into the callers.
Also remove `DIType::isValid()`, which is really just a check against
`nullptr` at this point.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235059 91177308-0d34-0410-b5e6-96231b3b80d8