[clang] [clang][Serialization] Outline LangOptions mismatch diagnostics (PR #202843)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I'm pretty impressed by the amount this shrinks our build (even if it is a drop in the bucket), and I suspect it doesn't have any corresponding performance impact.  

I'd like to see if others have concerns, but I'm leaning towards being OK with this.

https://github.com/llvm/llvm-project/pull/202843