GoObj: mark live stack-object pointer fields

[zhouguangyuan0718]

Depends on #38; the diff will reduce to the final commit after that PR lands.

An active address-taken stack object needs two complementary forms of GC metadata: function-wide StackObjects describe its layout for stack relocation, while the call-site LocalsPointerMaps keep its live heap-pointer fields reachable during marking. The writer previously emitted only StackObjects for this record kind, allowing GC to free objects still referenced from the stack object.

This change expands active StackObject bitmaps into LocalsPointerMaps while retaining the function-wide StackObjects record.

Validation:

• GoALLC plugin CTest: 57/57
• Test/LLVM/alloca-statepoint
• Test/LLVM/runtime/llvm_alloca_statepoint_gc.go
• Test/LLVM/runtime/fixedbugs/issue27518b.go
• LLVM defer/function-info verifier tests: 3/3

[github-actions]

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[zhouguangyuan0718]

Closing after full-matrix validation exposed that expanding StackObject fields directly into LocalsPointerMaps violates Go stack-object reachability: stackobj.go and stackobj3.go retained dead objects, and stack pointers could be relocated twice. The branch now reverts the change completely. The replacement fix is in goallc/go#75: it materializes a short-lived pointer root for each live direct StackObject address at the corresponding safepoint, preserving both issue27518b/alloca GC coverage and the existing stackobj liveness tests.