[llvm] [llvm][CFI] Ensure COFF comdat renaming applies for imported functions (PR #143421)

[via llvm-commits]

PiJoules wrote:

> How will this affect FatLTO? We run a version of LowerTypeTests as part of the non-LTO compilation that basically drops everything related to tests after we apply some cleanups (
> 
> https://github.com/llvm/llvm-project/blob/6cbd91ea52cbf2b27dc92557cea64f5fd305fcf1/llvm/lib/Passes/PassBuilderPipelines.cpp#L1694
> 
> ). I don't think the renaming will happen in this case, but I'm not 100% certain.

I think the renaming shouldn't happen since `LowerTypeTestsModule::lower` should immediately return if `DropTypeTests != DropTestKind::None` and all the renaming and actual lowering happens after that check.

https://github.com/llvm/llvm-project/pull/143421