[mlir] Allow Diagnostic/InFlightDiagnostic to convert to FailureOr.

[bzcheeseman]

Summary

Allow conversion of a diagnostic to FailureOr. This conversion only results
in failure because in the case where operator LogicalResult would return
success, the FailureOr constructor would assert.

mlir/include/mlir/IR/Diagnostics.h

public:
   }
   iterator_range<const_note_iterator> getNotes() const {
     return llvm::make_pointee_range(notes);
   }
 
   /// Allow a diagnostic to be converted to 'failure'.
   operator LogicalResult() const;
 
+  /// Allow a diagnostic to be converted to FailureOr<T>. Always results in
+  /// 'failure' because this cast cannot possibly return an object of 'T'.
+  template <typename T>
+  operator FailureOr<T>() const {
+    return failure();
+  }
+
 private:
   Diagnostic(const Diagnostic &rhs) = delete;
   Diagnostic &operator=(const Diagnostic &rhs) = delete;
 
   /// The source location.
   Location loc;
 
   /// The severity of this diagnostic.
public:
 
   /// Abandons this diagnostic so that it will no longer be reported.
   void abandon();
 
   /// Allow an inflight diagnostic to be converted to 'failure', otherwise
   /// 'success' if this is an empty diagnostic.
   operator LogicalResult() const;
 
+  /// Allow an inflight diagnostic to be converted to FailureOr<T>. Always
+  /// results in 'failure' because this cast cannot possibly return an object of
+  /// 'T'.
+  template <typename T>
+  operator FailureOr<T>() const {
+    return failure();
+  }
+
 private:
   InFlightDiagnostic &operator=(const InFlightDiagnostic &) = delete;
   InFlightDiagnostic &operator=(InFlightDiagnostic &&) = delete;
   InFlightDiagnostic(DiagnosticEngine *owner, Diagnostic &&rhs)
       : owner(owner), impl(std::move(rhs)) {}
 
   /// Returns true if the diagnostic is still active, i.e. it has a live
   /// diagnostic.