Specify argument to be `const` for `debugString()` helper (NFC)

This allows using this helper with rvalues.

mlir/include/mlir/Support/DebugStringHelper.h

@@ -24,7 +24,8 @@
 namespace mlir {
 
 // Simple helper function that returns a string as printed from a op.
-template <typename T> static std::string debugString(T &op) {
+template <typename T>
+static std::string debugString(const T &op) {
   std::string instr_str;
   llvm::raw_string_ostream os(instr_str);
   op.print(os);