[MSBuild] Allow abstract members to be removed for intersection

<PropertyGroup>
  <IntersectionRemoveAbstractTypeMembers>Type1;Type2</IntersectionRemoveAbstractTypeMembers>
</PropertyGroup>

Author: mrward

src/Build/NuGet.Build.Packaging.Tasks/ApiIntersect.cs

@@ -19,6 +19,8 @@ public class ApiIntersect : ToolTask
 
 		public ITaskItem[] ExcludeType { get; set; }
 
+		public ITaskItem[] RemoveAbstractTypeMembers { get; set; }
+
 		public string KeepInternalConstructors { get; set; }
 
 		public string KeepMarshalling { get; set; }
@@ -78,6 +80,14 @@ protected override string GenerateCommandLineCommands()
 				builder.AppendTextUnquoted("\"");
 			}
 
+			foreach (var removeAbstractType in RemoveAbstractTypeMembers.NullAsEmpty())
+			{
+				builder.AppendSwitch("-a");
+				builder.AppendTextUnquoted(" \"");
+				builder.AppendTextUnquoted(removeAbstractType.ItemSpec);
+				builder.AppendTextUnquoted("\"");
+			}
+
 			AppendSwitchIfTrue(builder, "-k", KeepInternalConstructors);
 
 			AppendSwitchIfTrue(builder, "-m", KeepMarshalling);

src/Build/NuGet.Build.Packaging.Tasks/NuGet.Build.Packaging.targets

@@ -523,6 +523,7 @@ Copyright (c) .NET Foundation. All rights reserved.
 			IntersectionAssembly="@(IntersectionAssembly)"
 			ReferencePath="$(IntersectionAssemblyReferencePath)"
 			ExcludeType="$(IntersectionExcludeType)"
+			RemoveAbstractTypeMembers="$(IntersectionRemoveAbstractTypeMembers)"
 			KeepInternalConstructors="$(IntersectionKeepInternalConstructors)"
 			KeepMarshalling="$(IntersectionKeepMarshalling)"
 			ToolPath="$(ApiIntersectToolPath)"