Build .NETCoreApp source gens in allconfigurations build (#105247)

* Build .NETCoreApp source gens in allconfigurations build

Fixes #100346

* Update RegexGenerator.Emitter.cs

---------

Co-authored-by: Matt Mitchell <mmitche@microsoft.com>

Author: ViktorHofer

src/libraries/oob-gen.proj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.Build.Traversal">
+
+  <!-- Reference all out-of-band generator projects. -->
+  <ItemGroup>
+    <ProjectReference Include="$(MSBuildThisFileDirectory)*\gen\**\*.*proj"
+                      Exclude="@(ProjectExclusions)" />
+  </ItemGroup>
+
+</Project>

src/libraries/oob.proj

@@ -18,6 +18,7 @@
 
   <ItemGroup Condition="'$(BuildTargetFramework)' != '$(NetCoreAppCurrent)' or '$(BuildTargetFramework)' == ''">
     <ProjectReference Include="oob-all.proj" SkipGetTargetFrameworkProperties="true" />
+    <ProjectReference Include="oob-gen.proj" />
   </ItemGroup>
 
   <!-- Support building reference projects only. -->