[SwiftBuild] Preserve duplicate archive members when SwiftBuild expands static archives for linking

Description

Follow-up from #773.

SwiftBuild's archive expansion path assumes archive member names are unique. Some static archives contain duplicate basenames, and collapsing them during extraction loses object files before the final link, which can show up as missing symbols or malformed outputs.

The attached reproducer is JavaScriptKit-agnostic. It regenerates a binary target archive with two object files that deliberately share the same archive member basename:

common.o
common.o

It then builds a static Wrapper product with both the native/control build system and SwiftBuild, and validates the produced wrapper archive with llvm-ar and llvm-nm.

Expected behavior

When an archive contains duplicate member names, every member should still be preserved during expansion for linking.

Duplicate member basenames should not cause object-file loss during link preparation, and final wasm products should link successfully when archives contain colliding member names.

Actual behavior

With swift-DEVELOPMENT-SNAPSHOT-2026-06-12-a_wasm, the native/control path succeeds and preserves the duplicate archive semantics externally. The native wrapper archive contains only the wrapper objects, while references to both dup_a and dup_b remain unresolved for the external binary archive:

native wrapper archive members:
Wrapper.swiftmodule.o
wrap.swift.o
native symbols include unresolved dup_a and dup_b

The same reproducer with --build-system swiftbuild succeeds as a command, but produces a corrupted libWrapper.a by expanding the duplicate-member archive and retaining only one of the duplicate object definitions:

swiftbuild wrapper archive members:
Wrapper.o
common.o
swiftbuild_dup_symbol_definitions=1

In the verified run, llvm-nm showed only dup_b from the duplicate-member archive in the SwiftBuild-produced wrapper archive.

Steps to reproduce

Updated reproducer zip: https://gist.githubusercontent.com/kateinoigakukun/ba21c654a675032aa77906877c906e53/raw/e7de0e5c875db5dbaf6a4390668ffc811ae51fa1/781-duplicate-archive-members-linux-2026-06-12.zip

The updated zip preserves the original repro.sh and includes repro-linux-2026-06-12.sh plus adaptation notes for the Linux snapshot run.

unzip 781-duplicate-archive-members.zip
cd 781-duplicate-archive-members

TOOLCHAIN=/home/ubuntu/.local/share/swiftly/toolchains/main-snapshot-2026-06-12 \
SWIFT_SDK=swift-DEVELOPMENT-SNAPSHOT-2026-06-12-a_wasm \
SYSROOT=/home/ubuntu/.swiftpm/swift-sdks/swift-DEVELOPMENT-SNAPSHOT-2026-06-12-a_wasm.artifactbundle/swift-DEVELOPMENT-SNAPSHOT-2026-06-12-a_wasm/wasm32-unknown-wasip1/WASI.sdk \
./repro-linux-2026-06-12.sh

On macOS with the original 2026-05-27 snapshot setup described in the original reproducer, ./repro.sh preserves the original invocation shape.

Swift Package Manager version/commit hash

Swift Package Manager - Swift 6.5.0-dev

Swift & OS version (output of swift --version ; uname -a)

Swift version 6.5-dev (LLVM 18d2bfb70c14d89, Swift c13d82e5987aecb)
Target: x86_64-unknown-linux-gnu
Build config: +assertions
Linux swift-dev.fugu.katei.dev 6.8.0-94-generic #96-Ubuntu SMP PREEMPT_DYNAMIC Fri Jan  9 20:36:55 UTC 2026 x86_64 x86_64 x86_64 GNU/Linux

首先解包連結的 reproducer 並執行 repro-linux-2026-06-12.sh，或在 macOS 上執行 repro.sh，然後使用 llvm-ar 比較 native 和 SwiftBuild archive 的成員，並使用 llvm-nm 比較符號。追蹤負責產生不同輸出的 SwiftBuild archive 展開路徑；完成的標準是兩個重複成員都在準備階段後保留下來，並且最終的 wasm 產物在 dup_a 和 dup_b 都存在的情況下完成連結。