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

[Matt Arsenault via cfe-commits]

arsenm 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 

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