Make ScopedDiagnosticHandler destructor virtual (NFC)

[mehdi_amini]

Summary

This will allow to support polymorphic destruction.

Fixes #59212

[stellaraccident]

Thanks for having a look. I left a note on the issue about possibly doing this differently. WDYT? https://github.com/llvm/llvm-project/issues/59212#issuecomment-1328315420

[stellaraccident]

(change lgtm and I think we may want to rework this API at some point)

mlir/include/mlir/IR/Diagnostics.h

 class ScopedDiagnosticHandler {
 public:
   explicit ScopedDiagnosticHandler(MLIRContext *ctx) : handlerID(0), ctx(ctx) {}
   template <typename FuncTy>
   ScopedDiagnosticHandler(MLIRContext *ctx, FuncTy &&handler)
       : handlerID(0), ctx(ctx) {
     setHandler(std::forward<FuncTy>(handler));
   }
-  ~ScopedDiagnosticHandler();
+  virtual ~ScopedDiagnosticHandler();
 
 protected:
   /// Set the handler to manage via RAII.
   template <typename FuncTy>
   void setHandler(FuncTy &&handler) {
     auto &diagEngine = ctx->getDiagEngine();
     if (handlerID)
       diagEngine.eraseHandler(handlerID);