[Codegen][CPU] End-to-end-through-codegen lit test for ukernel framework. (#24571)

Closes the loop on the new C-bitcode ukernel framework with an
`iree-opt` lit test that drives the full LLVMCPU codegen pipeline on an
`inner_tiled` rooted dispatch wrapped in the standard
`hal.executable.variant` + binding-subspan structure, and asserts the
whole chain end-to-end:

- Bitcode reaches the dispatch's `hal.executable.variant` as a
first-class `objects(...)` attribute (the long-lived home).
- `llvm.func @iree_uk_mma_…` is declared with `hal.import.bitcode =
true`, the flag that tells `RewriteExternCallOpToDynamicImportCallOp` to
skip the runtime-import-table indirection.
- A direct `llvm.call @iree_uk_mma_…` replaces the original
`inner_tiled` — no `__import_ordinal_iree_uk_mma_…` (which would mean we
accidentally re-used the legacy runtime-resolved path), no leftover
`inner_tiled`, no leftover `ukernel.generic`.

To make that chain work, two pieces had to land alongside the test:

* `UKernelProviderAttr::createAndReplaceWithUkernelOp` now actually
builds the `UKernelGenericOp` (returning `success`, not `nullopt`),
setting `fn_def_attrs = {hal.import.bitcode = true}` on it. That flag
propagates onto the `func.func` declaration that
`LowerUKernelOpsToCalls` synthesizes, and through it onto the
`llvm.func` that `ConvertToLLVM` produces. Without this, the
"self-contained bitcode" property of the new framework would not hold —
every external call would be routed through the runtime import table
just like the legacy mmt4d ukernels.

* Bitcode lookup-and-attach is factored into a public helper
`IREE::CPU::attachUKernelBitcodeOnOp`, called both from
`LLVMCPUSelectUKernels` (at kernel-config time, mirroring GPU's
`ensureUKernelBitcodeAndFinalizeConfig`) and from the provider itself
(as a backstop for the BYO / hand-written-IR paths). It attaches the
bitcode both as a `hal.executable.objects` discardable attribute on the
op (the GPU-aligned shape) *and* directly on the enclosing
`hal.executable.variant`'s `objects` operand attribute (the durable
home, sidestepping passes that strip discardable attrs through
`ConvertToLLVM`).

Progress towards https://github.com/iree-org/iree/issues/24574.

Signed-off-by: Benoit Jacob <jacob.benoit.1@gmail.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>