[llvm] [ThinLTO] Properly support targets that require importing all external functions (PR #133588)

[Shilei Tian via llvm-commits]

shiltian wrote:

`linkonce_odr` doesn't always work. For example, let's say we have a function `foo`, and it is imported into two modules `A` and `B`. Due to the different code in `A` and `B`, `foo` is optimized in different ways such that the two `foo` are no longer interchangeable.

With this being said, I think we need to mark all imported symbols as "internal" to avoid duplicate symbols.

https://github.com/llvm/llvm-project/pull/133588