[CIR] Add offload container attribute (#206576)
Introduce `cir.offload.container`, a CIR dialect unit attribute that marks an
MLIR module as holding a host CIR module and its associated device CIR
modules in one IR unit while later offload merge/split pipeline passes need
visibility into both sides.
The marked module's body holds nested `builtin.module` operations. Each
nested module is tagged with `cir.offload.kind`, represented by the new
`#cir.offload_kind<host>` / `#cir.offload_kind<device>` enum attribute.
The structural contract expected by the follow-up pipeline work is
enforced in `CIRDialect::verifyOperationAttribute`, following the
`gpu.container_module` precedent:
* `cir.offload.container` is a unit attribute attached to a
`builtin.module`
* the first nested module is the host module
* all following nested modules are device modules
* the body contains only nested `builtin.module` operations
* at least one device module is present
* `cir.offload.kind` is only ever attached to a `builtin.module`
This patch only introduces the IR representation and verifier tests. The
passes that create, consume, or split this container are left to follow-up
patches.
This is part of the Host-Device CIR Combine Pipeline work discussed on
Discourse:
[discourse.llvm.org/t/gsoc-2026-host-device-cir-combine-pipeline-project-idea-discussion/89623](https://discourse.llvm.org/t/gsoc-2026-host-device-cir-combine-pipeline-project-idea-discussion/89623)
