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

[kadir çetinkaya via cfe-commits]

kadircet wrote:

yes turning diagnosticoptions to be a ThreadSafeRefCountedBase pointer should ensure we have similar guarantees. so LG from clangd side, but 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.

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