Build fixes

* Update local copy of ExperimentalAttribute per dotnet/runtime#85444

* Deprecate this repo's option validator code gen in favor of the runtime'src

- This removes traces of the option code generator in this repo, now
that the code has moved to dotnet/runtime. Unfortunately, the generator
in dotnet/runtime has currently dropped the use of the 'file' visibility
accessor in generated code, which leads to some conflicting type
definitions in this code base due to the use of IntervalsVisibleTo.
This surfaces as build warnings which are safe to ignore, since the
C# compiler ends up binding to the right thing. Fixing these warnings
will require a new drop of the runtime's generator.

* Suppress CS0436

Directory.Build.targets

@@ -16,7 +16,17 @@
     <!-- This is false for local development, but set to true in the CI system -->
     <TreatWarningsAsErrors Condition=" '$(TreatWarningsAsErrors)' == '' ">false</TreatWarningsAsErrors>
 
-    <!-- TODO: to be reenabled and all warnings fixed (https://github.com/dotnet/r9/issues/177) -->
+    <!--
+      TODO: to be resolved (https://github.com/dotnet/extensions/issues/4128)
+
+      warning CS0436: (NETCORE_ENGINEERING_TELEMETRY=Build) The type '__Attributes' in
+      'Microsoft.Extensions.Options.SourceGeneration/Microsoft.Extensions.Options.Generators.Generator/Validators.g.cs' conflicts with the imported type
+      '__Attributes' in 'Microsoft.Extensions.Telemetry, Version=8.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.
+      Using the type defined in 'Microsoft.Extensions.Options.SourceGeneration/Microsoft.Extensions.Options.Generators.Generator/Validators.g.cs'.
+     -->
+    <NoWarn>$(NoWarn);CS0436</NoWarn>
+
+    <!-- TODO: to be re-enabled and all warnings fixed (https://github.com/dotnet/extensions/issues/4002) -->
     <NoWarn>$(NoWarn);IL2026;IL2087;IL2067;IL2075;IL2091;IL2072;IL2090;CA1825;IL2070;IL2098;IL2057</NoWarn>
 
     <!-- Prevent analyzer crashes from stopping things -->
@@ -35,14 +45,14 @@
     <NoWarn Condition="'$(TargetFramework)' == 'net462' or '$(TargetFramework)' == 'net472' or '$(TargetFramework)' == 'netstandard2.0'">$(NoWarn);CA1062</NoWarn>
   </PropertyGroup>
 
-  <!-- .NET 8 specific configurations (https://github.com/dotnet/r9/issues/177) -->
+  <!-- .NET 8 specific configurations (https://github.com/dotnet/extensions/issues/4126) -->
   <PropertyGroup Condition="'$(TargetFramework)' == 'net8.0'">
     <!-- Ignore ASP0019: Use IHeaderDictionary.Append or the indexer to append or set headers. IDictionary.Add will throw an ArgumentException when attempting to add a duplicate key  -->
     <NoWarn>$(NoWarn);ASP0019</NoWarn>
   </PropertyGroup>
 
   <PropertyGroup>
-    <!-- Ignore RS1024: Compare symbols correctly (https://github.com/dotnet/r9/issues/177) -->
+    <!-- Ignore RS1024: Compare symbols correctly (https://github.com/dotnet/extensions/issues/4127) -->
     <NoWarn>$(NoWarn);RS1024</NoWarn>
   </PropertyGroup>
 
@@ -66,7 +76,9 @@
   </Target>
 
   <ItemGroup Condition="'$(Stage)' == 'dev' AND '$(OutputType)' != 'Exe' AND '$(Api)' != 'false'">
-    <AssemblyAttribute Include="System.Diagnostics.CodeAnalysis.ExperimentalAttribute" />
+    <AssemblyAttribute Include="System.Diagnostics.CodeAnalysis.ExperimentalAttribute">
+      <_Parameter1>TBD</_Parameter1>
+    </AssemblyAttribute>
   </ItemGroup>
 
   <Target Name="AddInternalsVisibleToDynamicProxyGenAssembly2" BeforeTargets="BeforeCompile">

eng/MSBuild/Generators.props

@@ -7,10 +7,6 @@
     <ProjectReference Include="$(MSBuildThisFileDirectory)\..\..\src\Generators\Microsoft.Gen.EnumStrings\Roslyn4.0\Microsoft.Gen.EnumStrings.Roslyn4.0.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" Private="false" IncludeAssets="runtime; build; native; contentfiles; analyzers; buildtransitive" />
   </ItemGroup>
 
-  <ItemGroup Condition="'$(UseOptionsValidationGenerator)' == 'true' and '$(BuildingSlnForDocs)' != 'true'">
-    <ProjectReference Include="$(MSBuildThisFileDirectory)\..\..\src\Generators\Microsoft.Gen.OptionsValidation\Roslyn4.0\Microsoft.Gen.OptionsValidation.Roslyn4.0.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" Private="false" IncludeAssets="runtime; build; native; contentfiles; analyzers; buildtransitive" />
-  </ItemGroup>
-
   <ItemGroup Condition="'$(UseMeteringGenerator)' == 'true' and '$(BuildingSlnForDocs)' != 'true'">
     <ProjectReference Include="$(MSBuildThisFileDirectory)\..\..\src\Generators\Microsoft.Gen.Metering\Roslyn4.0\Microsoft.Gen.Metering.Roslyn4.0.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" Private="false" IncludeAssets="runtime; build; native; contentfiles; analyzers; buildtransitive" />
   </ItemGroup>