Remove `const` from `const T &&` in debugString() helper to make it a universal reference (NFC)

It broke lvalue arguments otherwise:

mlir/include/mlir/Support/DebugStringHelper.h

@@ -25,7 +25,7 @@ namespace mlir {
 
 // Simple helper function that returns a string as printed from a op.
 template <typename T>
-static std::string debugString(const T &&op) {
+static std::string debugString(T &&op) {
   std::string instr_str;
   llvm::raw_string_ostream os(instr_str);
   op.print(os);