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

[via cfe-commits]

================
@@ -107,6 +107,7 @@ class ASTUnit {
 
 private:
   std::unique_ptr<LangOptions> LangOpts;
+  std::shared_ptr<DiagnosticOptions> DiagOpts;
----------------
cor3ntin wrote:

Ouch, this is not great :smile: 
Can you leave a FIXME comment in the source? Thanks

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