Make GenContext cleanup pointers non-owning

[ASDAlexander77]

Summary

Implements the ownership half of item A1 from docs/MLIRGen-refactoring-review.md.

GenContext held raw owning pointers (cleanUps, cleanUpOps, passResult, state) freed manually via clean()/cleanState(), while the struct is copied ~48× and copies share those pointers — a leak on any early-return path and a latent double-delete if clean() ever ran through two copies.

• The two setup sites now stack-allocate the objects — the discovery run (SmallVectors + PassResult + state in mlirGenFunctionEntry's discovery block) and the per-function state counter — and GenContext only borrows pointers to them. Declaration order guarantees the owners outlive every context copy in the call subtree.
• GenContext::clean() keeps its real job — erasing IR collected during a dummy run and the dummy funcOp — but no longer frees memory; it clears the borrowed lists and detaches. cleanState() is removed entirely.
• Removed the stale no-op clean() call and commented-out allocations in mlirDiscoverAllDependencies (that context never allocated cleanup lists nor set funcOp, so the call did nothing).

Net effect: memory lifetime is structural (stack unwinding), while the three explicit clean() calls remain for their IR-erasure side effect only.

Test plan

• tslang rebuilt cleanly (vs2026 debug)
• New smoke test driving exactly the changed paths — function return-type discovery (dummy run with stack-owned PassResult) and closure capture (outerVariables) — JIT output 3 42 as expected
• Prior smoke tests for Refactor theModule handling in MLIRGenImpl #201–Remove std::function churn and wasted arena interning in MLIRGen #203 paths unchanged (42 15 42 3 1, 5 21 7 x, and the utility-type/spread/push suite)
• MLIRGenTests.exe: 12/12 pass

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