Optimize self-reference item removal in target execution

[Copilot]

The pattern <ItemType Remove="@(ItemType)" /> already had fast-path optimization during project evaluation but was doing expensive file system operations during target execution.

Changes

• ItemGroupIntrinsicTask.ExecuteRemove: Added early detection for self-reference pattern before calling FindItemsMatchingSpecification

  • When detected, directly converts items to list instead of expanding references, glob matching, and building HashSets
  • Handles whitespace and case-insensitive item type names

• Tests: Added coverage for whitespace, case sensitivity, many items, and items with metadata

Example

<Target Name="Clean">
  <ItemGroup>
    <!-- Previously: expensive expansion + file system operations -->
    <!-- Now: direct list conversion -->
    <TempFiles Remove="@(TempFiles)" />
  </ItemGroup>
</Target>

The optimization now applies consistently in both evaluation phase (LazyItemEvaluator) and execution phase (ItemGroupIntrinsicTask).

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Fast clear all items primitive</issue_title>
<issue_description>We should detect the pattern for clearing all items and make it super cheap and fast:

    <!-- Remove items which we will never again use - they just sit around taking up memory otherwise -->
    <ItemGroup>
      <_AllChildProjectItemsWithTargetPath Remove="@(_AllChildProjectItemsWithTargetPath)"/>
    </ItemGroup>

    <ItemGroup>
      <Foo Remove="@(Foo)" />
    </ItemGroup>
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Fast clear all items primitive #11896

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Hello @@copilot, I noticed that you’re changing an .swr file or any file under src/Package/MSBuild.VSSetup.. Please make sure to validate this change by an experimental VS insertion. This is accomplished by pushing to an exp/* branch, which requires write permissions to this repo.