[NFC][YAML] Add IO::error() (llvm#123475)

For llvm#123280

Author: vitalybuka

llvm/include/llvm/Support/YAMLTraits.h

@@ -819,6 +819,7 @@ class IO {
   virtual NodeKind getNodeKind() = 0;
 
   virtual void setError(const Twine &) = 0;
+  virtual std::error_code error() = 0;
   virtual void setAllowUnknownKeys(bool Allow);
 
   template <typename T>
@@ -1448,7 +1449,7 @@ class Input : public IO {
   ~Input() override;
 
   // Check if there was an syntax or semantic error during parsing.
-  std::error_code error();
+  std::error_code error() override;
 
 private:
   bool outputting() const override;
@@ -1631,6 +1632,7 @@ class Output : public IO {
   void scalarTag(std::string &) override;
   NodeKind getNodeKind() override;
   void setError(const Twine &message) override;
+  std::error_code error() override;
   bool canElideEmptySequence() override;
 
   // These are only used by operator<<. They could be private

llvm/lib/Support/YAMLTraits.cpp

@@ -750,6 +750,8 @@ void Output::scalarTag(std::string &Tag) {
 void Output::setError(const Twine &message) {
 }
 
+std::error_code Output::error() { return {}; }
+
 bool Output::canElideEmptySequence() {
   // Normally, with an optional key/value where the value is an empty sequence,
   // the whole key/value can be not written.  But, that produces wrong yaml