[mlir] Deprecate OpPrintingFlags(std::nullopt_t) (NFC) (#149546)

This patch deprecates OpPrintingFlags(std::nullopt_t) to avoid use of
std::nullopt outside the context of std::optional.

Author: kazutakahirata

mlir/include/mlir/IR/OpDefinition.h

@@ -115,7 +115,7 @@ class OpState {
   MLIRContext *getContext() { return getOperation()->getContext(); }
 
   /// Print the operation to the given stream.
-  void print(raw_ostream &os, OpPrintingFlags flags = std::nullopt) {
+  void print(raw_ostream &os, OpPrintingFlags flags = {}) {
     state->print(os, flags);
   }
   void print(raw_ostream &os, AsmState &asmState) {

mlir/include/mlir/IR/Operation.h

@@ -318,7 +318,7 @@ class alignas(8) Operation final
   /// take O(N) where N is the number of operations within the parent block.
   bool isBeforeInBlock(Operation *other);
 
-  void print(raw_ostream &os, const OpPrintingFlags &flags = std::nullopt);
+  void print(raw_ostream &os, const OpPrintingFlags &flags = {});
   void print(raw_ostream &os, AsmState &state);
   void dump();
 

mlir/include/mlir/IR/OperationSupport.h

@@ -1176,6 +1176,7 @@ class alignas(8) OperandStorage {
 class OpPrintingFlags {
 public:
   OpPrintingFlags();
+  LLVM_DEPRECATED("Use OpPrintingFlags() instead", "OpPrintingFlags()")
   OpPrintingFlags(std::nullopt_t) : OpPrintingFlags() {}
 
   /// Enables the elision of large elements attributes by printing a lexically