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

[Matt Arsenault via llvm-commits]

arsenm wrote:

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

There are 2 issues. The primary issue is reporting register and other resource usage. We need some kind of module summary in the linked module to avoid assuming an unacceptable worst case scenario in the presence of any external calls. 

A second issue is we don't have a way to codegen LDS which is referenced in external modules, but this is more of a niche usage that doesn't appear that often. 



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