[clang] [clang-tools-extra] [clang] Hide the `DiagnosticOptions` pointer from `CompilerInvocation` (PR #106274)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > yes turning diagnosticoptions to be a ThreadSafeRefCountedBase pointer should ensure we have similar guarantees. so LG from clangd side
> 
> Thanks for confirming!
> 
> > I am not sure about implications on using thread-aware structs in core parts of clang. so it might be worthwhile to get some opinions from clang maintainers as well.
> 
> Good point. @AaronBallman any thoughts?

So long as there are sufficient comments explaining why the thread-safety is needed, I think it's okay.

https://github.com/llvm/llvm-project/pull/106274