Add copyright + extracted common disposer.

src/Workspaces/Core/Portable/Utilities/ObjectPools/ArrayBuilder.cs

-using System;
+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 namespace Microsoft.CodeAnalysis.PooledObjects
 {
-    internal partial class ArrayBuilder<T>
+    internal partial class ArrayBuilder<T> : IPooled
     {
-        public static ArrayBuilderDisposer GetInstance(int capacity, out ArrayBuilder<T> instance)
-        {
-            instance = GetInstance(capacity);
-            return new ArrayBuilderDisposer(instance);
-        }
-
-        public static ArrayBuilderDisposer GetInstance(out ArrayBuilder<T> instance)
+        public static PooledDisposer<ArrayBuilder<T>> GetInstance(out ArrayBuilder<T> instance)
         {
             instance = GetInstance();
-            return new ArrayBuilderDisposer(instance);
+            return new PooledDisposer<ArrayBuilder<T>>(instance);
         }
 
-        public static ArrayBuilderDisposer GetInstance(int capacity, T fillWithValue, out ArrayBuilder<T> instance)
+        public static PooledDisposer<ArrayBuilder<T>> GetInstance(int capacity, out ArrayBuilder<T> instance)
         {
-            instance = GetInstance(capacity, fillWithValue);
-            return new ArrayBuilderDisposer(instance);
+            instance = GetInstance(capacity);
+            return new PooledDisposer<ArrayBuilder<T>>(instance);
         }
 
-        internal struct ArrayBuilderDisposer : IDisposable
+        public static PooledDisposer<ArrayBuilder<T>> GetInstance(int capacity, T fillWithValue, out ArrayBuilder<T> instance)
         {
-            private ArrayBuilder<T> _pooledObject;
-
-            public ArrayBuilderDisposer(ArrayBuilder<T> instance)
-            {
-                _pooledObject = instance;
-            }
-
-            public void Dispose()
-            {
-                var pooledObject = _pooledObject;
-                if (pooledObject != null)
-                {
-                    pooledObject.Free();
-                    _pooledObject = null;
-                }
-            }
+            instance = GetInstance(capacity, fillWithValue);
+            return new PooledDisposer<ArrayBuilder<T>>(instance);
         }
     }
 }

src/Workspaces/Core/Portable/Utilities/ObjectPools/IPooled.cs

+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+namespace Microsoft.CodeAnalysis.PooledObjects
+{
+    interface IPooled
+    {
+        void Free();
+    }
+}

src/Workspaces/Core/Portable/Utilities/ObjectPools/PooledDisposer.cs

+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+using System;
+
+namespace Microsoft.CodeAnalysis.PooledObjects
+{
+    internal struct PooledDisposer<TPoolable> : IDisposable
+        where TPoolable : IPooled
+    {
+        private TPoolable _pooledObject;
+
+        public PooledDisposer(TPoolable instance)
+        {
+            _pooledObject = instance;
+        }
+
+        public void Dispose()
+        {
+            var pooledObject = _pooledObject;
+            if (pooledObject != default)
+            {
+                pooledObject.Free();
+                _pooledObject = default;
+            }
+        }
+    }
+
+}

src/Workspaces/Core/Portable/Utilities/ObjectPools/PooledHashSet.cs

-using System;
+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 namespace Microsoft.CodeAnalysis.PooledObjects
 {
-    internal partial class PooledHashSet<T>
+    internal partial class PooledHashSet<T> : IPooled
     {
-        public static PooledHashSetDisposer GetInstance(out PooledHashSet<T> instance)
+        public static PooledDisposer<PooledHashSet<T>> GetInstance(out PooledHashSet<T> instance)
         {
             instance = GetInstance();
-            return new PooledHashSetDisposer(instance);
-        }
-
-        internal struct PooledHashSetDisposer : IDisposable
-        {
-            private PooledHashSet<T> _pooledObject;
-
-            public PooledHashSetDisposer(PooledHashSet<T> instance)
-            {
-                _pooledObject = instance;
-            }
-
-            public void Dispose()
-            {
-                var pooledObject = _pooledObject;
-                if (pooledObject != null)
-                {
-                    pooledObject.Free();
-                    _pooledObject = null;
-                }
-            }
+            return new PooledDisposer<PooledHashSet<T>>(instance);
         }
     }
 }

src/Workspaces/Core/Portable/Utilities/ObjectPools/PooledStringBuilder.cs

-using System;
+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 namespace Microsoft.CodeAnalysis.PooledObjects
 {
-    internal partial class PooledStringBuilder
+    internal partial class PooledStringBuilder : IPooled
     {
-        public static PooledStringBuilderDisposer GetInstance(out PooledStringBuilder instance)
+        public static PooledDisposer<PooledStringBuilder> GetInstance(out PooledStringBuilder instance)
         {
             instance = GetInstance();
-            return new PooledStringBuilderDisposer(instance);
-        }
-
-        internal struct PooledStringBuilderDisposer : IDisposable
-        {
-            private PooledStringBuilder _pooledObject;
-
-            public PooledStringBuilderDisposer(PooledStringBuilder instance)
-            {
-                _pooledObject = instance;
-            }
-
-            public void Dispose()
-            {
-                var pooledObject = _pooledObject;
-                if (pooledObject != null)
-                {
-                    pooledObject.Free();
-                    _pooledObject = null;
-                }
-            }
+            return new PooledDisposer<PooledStringBuilder>(instance);
         }
     }
 }