Devirtualize HttpHeaders.Reset (#5950)

* Use Inlining to devirtualize

src/Servers/Kestrel/Core/src/Internal/Http/HttpHeaders.cs

@@ -111,6 +111,8 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http
             _isReadOnly = true;
         }
 
+        // Inline to allow ClearFast to devirtualize in caller
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void Reset()
         {
             _isReadOnly = false;

src/Servers/Kestrel/Core/src/Internal/Http/HttpRequestHeaders.cs

@@ -11,7 +11,7 @@ using Microsoft.Net.Http.Headers;
 
 namespace Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http
 {
-    public partial class HttpRequestHeaders : HttpHeaders
+    public sealed partial class HttpRequestHeaders : HttpHeaders
     {
         private static long ParseContentLength(string value)
         {

src/Servers/Kestrel/Core/src/Internal/Http/HttpResponseHeaders.cs

@@ -12,7 +12,7 @@ using Microsoft.Net.Http.Headers;
 
 namespace Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http
 {
-    public partial class HttpResponseHeaders : HttpHeaders
+    public sealed partial class HttpResponseHeaders : HttpHeaders
     {
         private static readonly byte[] _CrLf = new[] { (byte)'\r', (byte)'\n' };
         private static readonly byte[] _colonSpace = new[] { (byte)':', (byte)' ' };