Make RequiredAttributeDescriptor IEquatable.

- All the other descriptors are already `IEquatable` this should be as well.
- Moved the required attribute descriptor comparer from Legacy => Language. It wasn't in the Legacy folder to begin with so this made sense given that it's already internal.

#1495

src/Microsoft.AspNetCore.Razor.Language/RequiredAttributeDescriptor.cs

@@ -9,7 +9,7 @@ using System.Linq;
 namespace Microsoft.AspNetCore.Razor.Language
 {
     [DebuggerDisplay("{DisplayName,nq}")]
-    public abstract class RequiredAttributeDescriptor
+    public abstract class RequiredAttributeDescriptor : IEquatable<RequiredAttributeDescriptor>
     {
         public string Name { get; protected set; }
 
@@ -33,6 +33,21 @@ namespace Microsoft.AspNetCore.Razor.Language
             }
         }
         
+        public bool Equals(RequiredAttributeDescriptor other)
+        {
+            return RequiredAttributeDescriptorComparer.Default.Equals(this, other);
+        }
+
+        public override bool Equals(object obj)
+        {
+            return Equals(obj as RequiredAttributeDescriptor);
+        }
+
+        public override int GetHashCode()
+        {
+            return RequiredAttributeDescriptorComparer.Default.GetHashCode(this);
+        }
+
         /// <summary>
         /// Acceptable <see cref="RequiredAttributeDescriptor.Name"/> comparison modes.
         /// </summary>

src/Microsoft.AspNetCore.Razor.Language/RequiredAttributeDescriptorComparer.cs

@@ -6,7 +6,7 @@ using System.Collections.Generic;
 using System.Linq;
 using Microsoft.Extensions.Internal;
 
-namespace Microsoft.AspNetCore.Razor.Language.Legacy
+namespace Microsoft.AspNetCore.Razor.Language
 {
     /// <summary>
     /// An <see cref="IEqualityComparer{TagHelperRequiredAttributeDescriptor}"/> used to check equality between