[mlir][debug] Handle DIImportedEntity. (#103055)

The `DIImporedEntity` can be used to represent imported entities like
C++'s namespace with using directive or fortran's moudule with use
statement.

This PR adds `DIImportedEntityAttr` and 2-way translation from
`DIImportedEntity` to `DIImportedEntityAttr` and vice versa.

When an entity is imported in a function, the `retainedNodes` field of
the `DISubprogram` contains all the imported nodes. See the C++ code and
the LLVM IR below.

```
void test() {
    using namespace n1;
 ...
}

!2 = !DINamespace(name: "n1", scope: null)
!16 = distinct !DISubprogram(name: "test", ..., retainedNodes: !19) !19 = !{!20}
!20 = !DIImportedEntity(tag: DW_TAG_imported_module, scope: !16, entity: !2 ...)
```

This PR makes sure that the translation from mlir to `retainedNodes`
field happens correctly both ways.

To side step the cyclic dependency between `DISubprogramAttr` and `DIImportedEntityAttr`,
we have decided to not have `scope` field in the `DIImportedEntityAttr` and it is inferred
from the entity which hold the list of `DIImportedEntityAttr`. A `retainedNodes` field has been
added in the `DISubprogramAttr` which contains the list of `DIImportedEntityAttr` for that
function.

This PR currently does not handle entities imported in a global scope
but that should be easy to handle in a subsequent PR.

0 files changed, 0 insertions, 0 deletions