Decompose 517-line mlirGen(ObjectLiteralExpression) into passes

[ASDAlexander77]

Summary

Second §5 item (giant-method decomposition) from docs/MLIRGen-refactoring-review.md, same treatment as cast() in #205.

mlirGen(ObjectLiteralExpression) interleaved three passes over the properties — data fields, method prototypes, method bodies — plus capture accumulation, glued together by five lambdas closing over shared accumulator vectors. Now:

• ObjectLiteralInfo struct holds the accumulated state (field infos, values, method refs, fields-to-set, receiver type, objThis, the literal node).
• Each pass is a named member function: mlirGenObjectLiteralFields → mlirGenObjectLiteralMethodPrototypes → mlirGenObjectLiteralCaptures → mlirGenObjectLiteralMethodBodies, with addObjectFieldInfo / addObjectFuncFieldInfo / processObjectFunctionLike(Proto) replacing the closures.
• The fields pass returns std::optional<ValueOrLogicalResult> so the early "success but no value" exits under allowPartialResolve propagate to the caller exactly as before — a plain LogicalResult would silently swallow them.
• Bodies moved verbatim; the driver is now ~90 lines and reads as the pass sequence. The union-receiver detection that narrows receiverType mid-pass now mutates oli.receiverType explicitly instead of a captured local.

Test plan

• tslang rebuilt cleanly (vs2026 debug)
• New smoke test covering every decomposed path — plain fields, shorthand, method with this access, arrow function capturing an outer variable (exercises the accumulated-captures field), getter, and object spread — JIT output 1 5 101 11 2 1 2 as expected
• All five prior smoke tests for Refactor theModule handling in MLIRGenImpl #201–Decompose 590-line cast() into named pipeline stages #205 paths unchanged
• MLIRGenTests.exe: 12/12 pass

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