[clang] [llvm] [Clang] Prevent `mlink-builtin-bitcode` from internalizing the RPC client (PR #118661)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> > clang attribute for externally_initialized
> 
> I'm surprised there isn't one already. Also seems better if you're going to special case the symbol to special case it by just setting this rather than skipping internalize for it

I'm not sure I want it in the generic case though, because I don't know the side-effects that LLVM attribute has. This is only necessary because of this stupid `mlink-builtin-bitcode` thing. if there's some other way to just tell it not to internalize this global that'd be great.

https://github.com/llvm/llvm-project/pull/118661