fix(ssa): preserve debug locations in synthetic builders

[cpunion]

Problem

Synthetic builders may not have a current source location even when their function has a DISubprogram. The LLVM binding explicitly requires a location to have been set before calling GetCurrentDebugLocation; deferInitBuilder violated that precondition and crashed in LLVMGoGetCurrentDebugLocation during CI.

Other synthetic calls in debug-enabled functions failed LLVM verification: an inlinable call must have a !dbg location. R4's macOS LTO checks exposed this for os.(*root).decref. The affected code exists on upstream main be23e488a; this PR contains no R4 runtime changes.

Fix

• Track explicitly assigned source locations in the Go builder instead of reading a potentially unset LLVM location.
• Initialize synthetic builders in debug-enabled functions with a line-zero function location, denoting generated code without inventing a source line.
• Preserve the originating source location for generated defer setup when available.

Validation

• Full SSA tests pass in R4; focused tests also pass on this main-based branch.
• Regressions cover builders created before/after debug initialization, debug-disabled functions, exact source-location inheritance, and LLVM verification of an inlinable synthetic call.
• Actual R4 native fixture tests built with debug info pass.
• The three failing macOS LTO cases advance past the original LLVM verifier error locally. Final local linking is blocked by this machine's LLVM 22 plugin / LLD 23 mismatch; this is not counted as an end-to-end pass. Matching LLVM 22 fork CI remains required.

No debug-info disabling or LLVM dependency replacement is used. Keep this draft until CI verification completes.

[cpunion]

Moved to xgo-dev#2529 as requested. All 8 remaining fork workflow runs are cancelled; verification continues in the upstream contribution PR. Keeping the source branch for that PR and the dependent stack-slot fix.