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

[Fangrui Song via llvm-commits]

MaskRay wrote:

https://reviews.llvm.org/D99683 added `-force-import-all` for AMDGPU to import external functions. 

> AMDGPU target does not support codegen of object files containing call of external functions, therefore the LLVM module passed to AMDGPU backend needs to contain definitions of all the callees.

Why is there such a limitation and would it be better to lift the limitation?

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