debug: preserve defer locations under full LTO

[cpunion]

Problem

Conditional defer lowering splits the function entry block with a fresh LLVM builder. In a function carrying DWARF, that builder does not inherit the Go source location that triggered defer setup, so synthesized calls such as GetThreadDefer, AllocU, SetThreadDefer, and Rethrow have no !dbg attachment.

Normal native links may accept that IR, but full LTO verifies the combined debug-bearing module and aborts with inlinable function call in a function with debug info must have a !dbg location.

Changes

• pass the source builder into defer-entry splitting
• copy its current DebugLoc to the synthetic builder when the function has debug metadata
• leave non-debug defer lowering unchanged
• add focused location-inheritance coverage
• add separate DWARF acceptance tests for built-in full LTO and the optional LTO pass plugin

The LTO tests disable LLGo's package cache so #2119's cold/warm metadata issue cannot hide or reintroduce this verifier failure.

Fixes #2118.

Verification

• built-in full LTO DWARF fixture on macOS and Ubuntu arm64
• pass-plugin full LTO DWARF fixture on Ubuntu arm64
• full ssa tests on macOS
• standards-based DWARF matrix on macOS and Ubuntu arm64
• changed helper coverage: 100%

Ubuntu validation used 2 CPUs, a 15 GiB memory limit, 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.

[cpunion]

Consolidated into #2143, which now contains the defer-call debug-location preservation and its LTO regression coverage. Closing this redundant PR; no code is being merged to main by this action.