Add 3pn notice for dotnet/corefx

- Due to https://github.com/dotnet/corefx/blob/master/src/System.Reflection.Metadata/src/System/Reflection/Internal/Utilities/ImmutableByteArrayInterop.cs

THIRD-PARTY-NOTICES

@@ -0,0 +1,36 @@
+.NET Core uses third-party libraries or other resources that may be
+distributed under licenses different than the .NET Core software.
+
+In the event that we accidentally failed to list a required notice, please
+bring it to our attention. Post an issue or email us:
+
+           dotnet@microsoft.com
+
+The attached notices are provided for information only.
+
+License notice for dotnet/corefx
+--------------------------------
+
+https://github.com/dotnet/corefx/blob/master/LICENSE
+
+The MIT License (MIT)
+
+Copyright (c) .NET Foundation and Contributors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

src/Microsoft.DiaSymReader.PortablePdb/Utilities/ImmutableByteArrayInterop.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+// copy of https://github.com/dotnet/corefx/blob/master/src/System.Reflection.Metadata/src/System/Reflection/Internal/Utilities/ImmutableByteArrayInterop.cs
+
 using System.Collections.Immutable;
 using System.Runtime.InteropServices;