Enable building WPF in the unified build (#18577)

src/SourceBuild/content/repo-projects/Directory.Build.targets

@@ -33,7 +33,6 @@
   <!-- Exclude repositories that currently don't build when not building source-only. -->
   <ItemGroup Condition="'$(DotNetBuildSourceOnly)' != 'true'">
     <RepositoryReference Remove="roslyn" />
-    <RepositoryReference Remove="wpf" />
     <RepositoryReference Remove="msbuild" />
     <RepositoryReference Remove="aspnetcore" />
     <RepositoryReference Remove="razor" />
@@ -419,6 +418,15 @@
 
   <Target Name="CleanupRepo"
           Condition="'$(CleanWhileBuilding)' == 'true' and Exists('$(ProjectDirectory)artifacts')">
+
+    <!--
+      Some repositories (WinForms) use source generators that open files manually and keep file handles open on the compiler server (CsWin32).
+      These source generators are written incorrectly (a source generator should never do IO itself),
+      but the required features to read in binary files as input to a source generator don't exist.
+      To work around these source generators, shut down the compiler server so we can delete the obj directories.
+    -->
+    <Exec Command="$(DotnetTool) build-server shutdown --vbcscompiler" />
+
     <PropertyGroup>
       <BuildLogsDir>$([MSBuild]::NormalizeDirectory('$(ProjectDirectory)', 'artifacts', 'buildLogs'))</BuildLogsDir>
       <BuildObjDir>$([MSBuild]::NormalizeDirectory('$(ProjectDirectory)', 'artifacts', 'buildObj'))</BuildObjDir>

src/SourceBuild/patches/winforms/0001-Don-t-error-when-file-copy-fails-due-to-locking-unti.patch

@@ -0,0 +1,26 @@
+From a91eef2a8e4c53f4674b24ff162622f3c80c054d Mon Sep 17 00:00:00 2001
+From: Jeremy Koritzinsky <jekoritz@microsoft.com>
+Date: Fri, 9 Feb 2024 10:59:55 -0800
+Subject: [PATCH] Don't error when file copy fails due to locking until all the
+ backoffs have failed
+
+Backport: https://github.com/dotnet/winforms/pull/10858
+---
+ eng/SourceBuild.props | 1 +
+ 1 file changed, 1 insertion(+)
+
+diff --git a/eng/SourceBuild.props b/eng/SourceBuild.props
+index 0d0194fb6..e951a31a0 100644
+--- a/eng/SourceBuild.props
++++ b/eng/SourceBuild.props
+@@ -4,5 +4,6 @@
+   <PropertyGroup>
+     <GitHubRepositoryName>winforms</GitHubRepositoryName>
+     <SourceBuildManagedOnly>true</SourceBuildManagedOnly>
++    <InnerBuildArgs>$(InnerBuildArgs) -warnNotAsError:MSB3026</InnerBuildArgs>
+   </PropertyGroup>
+ </Project>
+\ No newline at end of file
+-- 
+2.43.0.windows.1
+