[clang] [llvm] [LLVM][NFC] Use `used`'s element type if available (PR #116804)

[Alex Voicu via cfe-commits]

https://github.com/AlexVlx commented:

> Mind adding a test case? Perhaps something like the embed-bitcode.ll, but instead checking the round-tripping of compiler.used ?  

Done, although slightly differently; it does reflect that when there is a pre-existing compiler.used its element type is now retained.

https://github.com/llvm/llvm-project/pull/116804