[Triton] Construct stream.executable from Triton functions

[ezhulenev]

Depends on: #142

Instead of building hal.executable.source with PTX it should be possible to build stream.executable with nested module with LLVM IR. In this case Triton plugin will not have to make sure Triton ABI matches IREE HAL ABI. Triton kernel will become somewhat like a ukernel (see example in iree-org/iree#14101 (comment))

Discussion: https://discord.com/channels/689900678990135345/804403964463546378/1118690243672031264