[MLIR][Transform] Make TransformState constructor public

This is discussed in RFC:
https://discourse.llvm.org/t/rfc-making-the-constructor-of-the-transformstate-class-protected/80377

Author: kaitingwang

mlir/include/mlir/Dialect/Transform/Interfaces/TransformInterfaces.h

@@ -476,10 +476,6 @@ class TransformState {
     extensions.erase(TypeID::get<Ty>());
   }
 
-private:
-  /// Identifier for storing top-level value in the `operations` mapping.
-  static constexpr Value kTopLevelValue = Value();
-
   /// Creates a state for transform ops living in the given region. The second
   /// argument points to the root operation in the payload IR being transformed,
   /// which may or may not contain the region with transform ops. Additional
@@ -488,6 +484,10 @@ class TransformState {
                  const RaggedArray<MappedValue> &extraMappings = {},
                  const TransformOptions &options = TransformOptions());
 
+private:
+  /// Identifier for storing top-level value in the `operations` mapping.
+  static constexpr Value kTopLevelValue = Value();
+
   /// Returns the mappings frame for the region in which the value is defined.
   /// If `allowOutOfScope` is set to "false", asserts that the value is in
   /// scope, based on the current stack of frames.