[clang] [Clang][LTO] Assign GUIDs after post-opt bitcode linking (PR #211155)

[Shilei Tian via cfe-commits]

shiltian wrote:

* #211155 👈
* `main`

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This stack of pull requests is managed by <a href="https://github.com/shiltian/sgh"><b>sgh</b></a>.
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https://github.com/llvm/llvm-project/pull/211155