Fix intellisense package xml file failing up-to-date check in libs pr…

…ojects (dotnet#105243)

Fixes dotnet#92940

eng/intellisense.targets

@@ -7,6 +7,8 @@
     <IntellisensePackageXmlFilePath Condition="'$(IntellisensePackageXmlFilePath)' == '' and Exists($(IntellisensePackageXmlFilePathFromNetFolder))">$(IntellisensePackageXmlFilePathFromNetFolder)</IntellisensePackageXmlFilePath>
     <IntellisensePackageXmlFilePath Condition="'$(IntellisensePackageXmlFilePath)' == '' and Exists($(IntellisensePackageXmlFilePathFromDotNetPlatExtFolder))">$(IntellisensePackageXmlFilePathFromDotNetPlatExtFolder)</IntellisensePackageXmlFilePath>
 
+    <IntermediateDocFileItemFromIntellisensePackage>$(IntermediateOutputPath)$(TargetName).intellisense-package.xml</IntermediateDocFileItemFromIntellisensePackage>
+
     <!-- Suppress "CS1591 - Missing XML comment for publicly visible type or member" compiler errors when the intellisense package xml file is used. -->
     <NoWarn Condition="'$(SkipIntellisenseNoWarnCS1591)' != 'true'">$(NoWarn);1591</NoWarn>
   </PropertyGroup>
@@ -25,13 +27,26 @@
     <PackageDownload Include="Microsoft.Private.Intellisense" Version="[$(MicrosoftPrivateIntellisenseVersion)]" />
   </ItemGroup>
 
+  <!-- Prepare the intellisense package xml file by copying it to the project's intermediate folder and update its file timestamp.
+       This is necessary so that all project outputs are newer than all project inputs. Directly copying from the intellisense package
+       would violate that and break fast up-to-date check. -->
+  <Target Name="PrepareIntellisensePackageXmlFile"
+          Inputs="$(IntellisensePackageXmlFilePath)"
+          Outputs="$(IntermediateDocFileItemFromIntellisensePackage)">
+    <Copy SourceFiles="$(IntellisensePackageXmlFilePath)"
+          DestinationFiles="$(IntermediateDocFileItemFromIntellisensePackage)" />
+
+    <Touch Files="$(IntermediateDocFileItemFromIntellisensePackage)" />
+  </Target>
+
   <!-- Replace the compiler generated xml file in the obj folder with the one that comes from the intellisense package. -->
   <Target Name="ChangeDocumentationFileForPackaging"
+          DependsOnTargets="PrepareIntellisensePackageXmlFile"
           BeforeTargets="CopyFilesToOutputDirectory;DocumentationProjectOutputGroup"
           Condition="'$(UseCompilerGeneratedDocXmlFile)' != 'true' and '$(IntellisensePackageXmlFilePath)' != ''">
     <ItemGroup>
       <DocFileItem Remove="@(DocFileItem)" />
-      <DocFileItem Include="$(IntellisensePackageXmlFilePath)" />
+      <DocFileItem Include="$(IntermediateDocFileItemFromIntellisensePackage)" />
     </ItemGroup>
   </Target>