Continue gutting the `DIDescriptor` hierarchy. In this case, move the
guts of `DIScope::getName()` and `DIScope::getContext()` to
`MDScope::getName()` and `MDScope::getScope()`.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234691 91177308-0d34-0410-b5e6-96231b3b80d8