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

[Yaxun Liu via llvm-commits]

yxsamliu wrote:

> Functions with `available_externally` linkage are never emitted into the object file. They're eliminated later in the pipeline, which eventually leads to an undefined symbol error.

what if the same callee is called by more than one callers in two different object files? If global linkage is used, will that cause duplicate symbols in the final executable? Can we use linkonce_odr linkage instead so that duplicate symbols are allowed or merged?

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