[mlir] Add nodiscard attribute to allowsUnregisteredDialects (llvm#105530)

This getter can easily be confused with the similar named
allowUnregisteredDialects setter

Author: jorickert

mlir/include/mlir/IR/MLIRContext.h

@@ -133,7 +133,7 @@ class MLIRContext {
   Dialect *getOrLoadDialect(StringRef name);
 
   /// Return true if we allow to create operation for unregistered dialects.
-  bool allowsUnregisteredDialects();
+  [[nodiscard]] bool allowsUnregisteredDialects();
 
   /// Enables creating operations in unregistered dialects.
   /// This option is **heavily discouraged**: it is convenient during testing