[Based on #2143] cabi: preserve debug variable homes

[cpunion]

Depends on #2143.

Problem

LLGo currently disables optimized C ABI lowering whenever DWARF is enabled. The lowering replaces a source parameter's stack home with its ABI storage, but its selective operand rewrite does not update LLVM debug records. Enabling the lowering as-is can therefore leave dbg.declare attached to stale storage.

The dependency on #2143 is required by the end-to-end path. On latest main, the focused operand-rewrite test passes, but the real abi2 + DWARF large-aggregate test crashes while LLVM emits the object. With #2143's DWARF-safe optimization pipeline, the same test completes on both supported validation platforms.

Changes

• run the same C ABI transformer with and without DWARF
• replace all uses of the old parameter home so LLVM updates debug records as well as executable instructions
• restore only setup instructions before the parameter store to the original alloca
• add a focused LLVM-level regression that verifies both the preserved setup operand and the rewritten dbg.declare/load operands
• run the existing large-aggregate test/go probe in DWARF-enabled abi2 mode

The change remains inside the existing C ABI transformer; no debug-specific ABI representation or target exception is introduced.

Verification

• macOS arm64: full internal/cabi and internal/build package tests; real TestLargeArrayReturnDWARF passes in 15.10s
• Ubuntu arm64: focused debug-record regression and real TestLargeArrayReturnDWARF pass in 15.31s
• direct-main dependency check: the focused unit test passes, while the real acceptance test reproduces the LLVM object-emission crash

Ubuntu validation used 2 CPUs, an 8 GiB memory limit with no swap, and 512 PIDs.

[gemini-code-assist]

Caution

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.