[clang] [clang-tools-extra] [lldb] [clang] Remove intrusive reference count from `DiagnosticOptions` (PR #139584)

[Michael Spencer via cfe-commits]

https://github.com/Bigcheese approved this pull request.

lgtm. I agree this makes the lifetime clearer.

https://github.com/llvm/llvm-project/pull/139584