FIR operations that use derived types need to have type descriptor
globals available on device when offloading. Examples of this can be
seen in `CUFDeviceGlobal` which ensures that such type descriptor uses
work on device for CUF.
Similarly, this is needed for OpenACC. This change introduces a new
interface to the OpenACC dialect named
`IndirectGlobalAccessOpInterface` which can be attached to operations
that may result in generation of accesses that use type descriptor
globals. This functionality is needed for the `ACCImplicitDeclare` pass
that is coming in a follow-up change which implicitly ensures that all
referenced globals are available in OpenACC compute contexts.
The interface provides a `getReferencedSymbols` method that collects all
global symbols referenced by an operation. When a symbol table is
provided, the implementation for FIR recursively walks type descriptor
globals to find all transitively referenced symbols.
Note that alternately this could have been implemented in different
ways:
- Codegen could implicitly generate such type globals as needed by
changing the technique that relies on populating them during lowering
(eg generate them directly in gpu.module during codegen).
- This interface could attach to types instead of operations for a
potentially more conservative implementation which maps all type
descriptors even if the underlying implementation using it won't
necessarily need such mapping.
The technique chosen here is consistent with `CUFDeviceGlobal` (which
walks operations inside `prepareImplicitDeviceGlobals`) and avoids
conservative mapping of all type descriptors.