[llvm] [ThinLTOBitcodeWriter] Do not crash on a typed declaration (PR #69564)

[Igor Kudrin via llvm-commits]

igorkudrin wrote:

> What configuration provoked this failure?

The problem is probably not something that can be seen in the standard usage. We came across it while working on a downstream tool that runs some optimizations, including the LTO pipeline, and saves bitcode output files for later use. In our case, we can avoid the issue by excluding the `EliminateAvailableExternally` from the pipeline, but I think that the issue is worth fixing anyway, since the crash is triggered by a correct input IR.

https://github.com/llvm/llvm-project/pull/69564