[clang] [llvm] [ThinLTO] Support dead RTTI data elimination under -fno-split-lto-unit (PR #126336)

[Teresa Johnson via llvm-commits]

https://github.com/teresajohnson commented:

Thanks for sending the PR! I think this should enable some nice size savings hopefully.

I went through most of it (though not the bitcode or llvm assembler changes yet). A couple of concerns:
- It doesn't look like it is guarded on whole program visibility? Maybe I missed that check. See WholeProgramVisibilityEnabledInLTO in LTO.cpp.
- It would be good to avoid having to add the CXXABI handling in LLVM and the TargetTriple/OS checks in LTO - I've added some suggestions on how we can perhaps do that below in the code.
- I'd like to avoid removing references from the index, suggestion on that in the code too.

https://github.com/llvm/llvm-project/pull/126336