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

[Teresa Johnson via llvm-commits]

teresajohnson wrote:

Looks like this flag was added several years ago for AMDGPU (https://reviews.llvm.org/D99683). Do you know why it was sufficient then but not now?

While this was originally added for AMDGPU I'm concerned about changing the linkage type as other use cases of this option may have appeared, and this is inconsistent with the linkage type normally used for ThinLTO importing, and would result in linker multiply defined errors in the usual case. If you aren't able to link objects, why not just use full LTO, which is what this essentially becomes?

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