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

[Shilei Tian via cfe-commits]

shiltian wrote:

> Seems fine, `-mlink-builtin-bitcode` is already a hack and this just adds to it.

Right. I have no idea why we want to link into some bitcode libraries **after** the optimization. There must be a reason. @lamb-j might know more history.

https://github.com/llvm/llvm-project/pull/211155