llvm: fix duplicate archive member LTO conversion

[brendan-nasa]

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

I drafted this PR in conversation with a combination GPT 5.6 Sol, Sonnet 5, and Opus 5. By hand, I verified the root cause, the behavior of the ruby code, compared it to the new behavior, and verified the fix worked correctly. The project that depends on libClangDriver.a also now correctly links.

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Static archives can contain multiple members with the same name. Extracting them into one directory overwrites all but one, while llvm-ar r updates only the first matching member. This can leave bitcode behind and replace one translation unit with another.

Extract members separately, rebuild archives after conversion, and add regression coverage for duplicate member names.

Closes #301252.