radlink: prefer non-zero function prefixes for COMDAT Any

[honkstar1]

Summary

Prefer COMDAT Any definitions with non-zero prefix data over definitions with no prefix or an all-zero prefix. LLVM places prefix data before the function entry point, whose COFF symbol value gives the prefix length. AutoRTFM uses this data to find the closed function variant; selecting an uninstrumented duplicate can discard that information.

• Inspect only bytes before the symbol, reject out-of-range prefix lengths, and ignore uninitialized section data.
• Preserve deterministic input-order selection when both candidates have the same prefix status. Handle both replacement directions, since insertion order need not match input order.
• Leave Null, Largest, SameSize, and Any/Largest promotion unchanged.
• Add 44 regression combinations covering both input orders, 1/4 workers, zero/non-zero prefixes, invalid lengths, and unaffected selection modes.

This is a standalone change against dev (47de6bf), not a restacking of the existing optimization PRs.

Validation

• Clang/LLD 20.1.8 release (-O2) build.
• New regression fails with unmodified dev and passes with this change.
• Upstream linker suite: 141 passed, 0 failed/crashed, 1 excluded. import_export fails its export-offset assertion both before and after this patch in this environment; the baseline failure was reproduced separately.
• Deployment-baseline backport: 47a6150f, preserving the previously submitted 348ae713 stack. Oodle-enabled optimized build: 181 passed, 0 failed/crashed, 1 excluded in the linker/torture/base/eval2 selection. The excluded case is ms_link_icf_section_flag_eligibility, a Microsoft-linker comparison. Debugger and RDI suites were not part of that selection.
• The new regression also fails with the actual previously submitted 348ae713 binary.

No production binaries have been replaced, and no Engine/runtime or farm soak result is claimed.