lto: analyze aggregate MethodByName string arguments

[cpunion]

Unblocks #2143.

Problem

The MethodByName LTO pass assumed that every marked Go string argument had been lowered into separate (ptr, len) parameters. LLGo's arm64 LP64 C ABI path can instead keep the 16-byte Go string as a {ptr, len} aggregate. In that form the pass could not recover direct, concatenated, sliced, or forwarded constant names, so it left the generic reflect marker in place and prevented the intended named-method pruning.

Changes

• consume one logical string argument in either aggregate or split ABI form
• reuse that decoding for direct MethodByName calls, runtime.StringCat, runtime.StringSlice2, and forwarded function parameters
• preserve the generic marker whenever a name cannot be resolved
• add Go fixtures covering direct, concat, slice, forwarding, linked execution, actual unused-method pruning, and unknown-name fallback
• derive the aggregate-ABI assertions from generated Go modules rather than hand-written LLVM IR

The focused aggregate test generates frontend IR with automatic C ABI lowering disabled, applies LLGo's arm64 C ABI transformer, and runs the LTO pass. One module checks the four known-name markers without a generic value marker. A separate module checks conservative unknown-name fallback because a generic marker intentionally retains all matching methods and would invalidate an unused-method symbol assertion. The normal plugin runtime and symbol suites exercise the linked split-ABI path and verify that Known.Drop is absent from the final binary.

This PR changes analysis only; it does not change either ABI.

Validation

• negative check: the Go-generated aggregate test fails with the main plugin at the missing Concat marker and passes with this PR
• macOS arm64, LLVM 19: clean plugin build and both generated aggregate modules pass
• Ubuntu arm64 container, capped at 2 CPUs / 15 GiB: both generated aggregate modules pass
• Ubuntu arm64: linked runtime fixture passes and the final symbol table excludes Known.Drop
• Ubuntu arm64: complete existing LTO-plugin runtime and symbol suites; the new fixture passes in the runtime suite
• Ubuntu arm64: CI-equivalent plugin coverage command, 52.7% aggregate coverage for the instrumented Go packages
• prior CI: Dev LTO GlobalDCE, both coverage platforms, and Codecov patch coverage pass

[gemini-code-assist]

Caution

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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