radlink: honor explicitly supplied ASan runtime libraries

[honkstar1]

Summary

Honor already-requested ASan runtime libraries instead of inferring a second, differently named SDK runtime.

A FortniteClient ASan link explicitly supplies EpicClang's clang_rt.asan_dynamic.lib and clang_rt.asan_dynamic_runtime_thunk.lib. The old inference path additionally requests the MSVC *-x86_64.lib variants and whole-archives the SDK thunk. Loading both produces multiply-defined __asan_globals_start and weak-registration symbols.

Changes

• Recognize requested components using either Windows naming convention: unsuffixed names in architecture-specific directories, or the existing architecture-suffixed names.
• Reuse those requests instead of introducing another runtime.
• If only one component is supplied, infer its matching companion and naming convention, preferring the supplied component's directory and then normal LIBPATH lookup.
• Keep existing SDK defaults when neither component was requested.
• Continue whole-archiving the selected thunk, preserving otherwise unreferenced registration members.
• Preserve the dynamic/static CRT distinction and optional VC-ASan support libraries. Do not remove user inputs or suppress genuine multiply-defined-symbol errors.

The lookup runs only during ASan inference, not per object or relocation.

Regression coverage

Synthetic COFF libraries exercise 12 positive combinations: dynamic/static CRTs, explicit pairs, either component alone, unchanged defaults, explicit canonical names, and companion fallback through LIBPATH. Paths with spaces and whole-archive registration are checked. A negative case explicitly loads conflicting thunks and must still report the duplicate.

Validation

• Standalone against dev@47de6bf3, without the optimization/compressed-object stack.
• Clang/LLD 20.1.8 Release build; upstream linker suite: 141 passed, 0 failed/crashed, 1 excluded (import_export, whose assertion was separately reproduced on the unmodified upstream baseline).
• The new regression reproduces Error(024) with the previous 5f2c8d69 linker and passes with this change.
• Equivalent combined candidate 0ff079a6: 184 passed, 0 failed/crashed, 1 existing Microsoft-linker-comparison exclusion. It retains the earlier radlink: match MSVC for discarded associative COMDAT relocations #940 associative-metadata correction.
• The supplied FortniteClient response reproduced the two reported duplicate definitions on 5f2c8d69; the fixed combined linker completes it with no diagnostics. The generated image imports clang_rt.asan_dynamic.dll, not the SDK's second ASan DLL. Original response and input size/timestamp metadata remained unchanged; outputs were redirected to an isolated directory.

No proprietary Engine objects/libraries are added as fixtures. The game executable was not launched; no runtime ASan/farm/UBA/Wine or performance result is claimed. No installed Engine linker is replaced by this PR.