[MLIR][LLVM] Debug info: fix DIDerivedTypeAttr relation to DIScopeAttr (#138200)
Fix `di_subprogram` references to scopes composed of `di_derived_type`s,
which currently fail to parse with:
```
error: failed to parse LLVM_DISubprogramAttr parameter 'scope' which is to be a `DIScopeAttr`
```