Simplify TrimmerSingleWarn intermediate assembly update using MSBuild item Update

[Copilot]

Description

Follow-up to PR #124801 review feedback: the "intersection via include/remove, then remove+re-include" pattern in _PrepareTrimConfiguration was complex, mutated item ordering, and used two throwaway item groups. Replace with a direct MSBuild Update.

Change

Before — compute intersection manually to set metadata, then remove+re-add items:

<__SingleWarnIntermediateAssembly Include="@(ResolvedFileToPublish)" />
<__SingleWarnIntermediateAssembly Remove="@(IntermediateAssembly)" />
<_SingleWarnIntermediateAssembly Include="@(ResolvedFileToPublish)" />
<_SingleWarnIntermediateAssembly Remove="@(__SingleWarnIntermediateAssembly)" />
<_SingleWarnIntermediateAssembly>
  <TrimmerSingleWarn Condition="...">false</TrimmerSingleWarn>
</_SingleWarnIntermediateAssembly>
<ResolvedFileToPublish Remove="@(_SingleWarnIntermediateAssembly)" />
<ResolvedFileToPublish Include="@(_SingleWarnIntermediateAssembly)" />

After — update matching items directly, preserving order:

<ResolvedFileToPublish Update="@(IntermediateAssembly)">
  <TrimmerSingleWarn Condition=" '%(ResolvedFileToPublish.TrimmerSingleWarn)' == '' ">false</TrimmerSingleWarn>
</ResolvedFileToPublish>

Changes proposed in this pull request

• [Microsoft.NET.ILLink.targets] Replace 13-line intersection pattern with a 3-line Update in _PrepareTrimConfiguration
• [Microsoft.NET.ILLink.targets] Qualify %(TrimmerSingleWarn) as %(ResolvedFileToPublish.TrimmerSingleWarn) in the Update condition to prevent MSB4096 (unqualified metadata batching over all ResolvedFileToPublish items, including those without the metadata defined)

Additional context

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[sbomer]

@copilot fix this error:

##[error]src/tools/illink/src/ILLink.Tasks/build/Microsoft.NET.ILLink.targets(227,5): error MSB4096: (NETCORE_ENGINEERING_TELEMETRY=Build) The item "/__w/1/s/artifacts/obj/coreclr/ILCompiler_inbuild/linux.x64.Release/singlefilehost" in item list "ResolvedFileToPublish" does not define a value for metadata "TrimmerSingleWarn".  In order to use this metadata, either qualify it by specifying %(ResolvedFileToPublish.TrimmerSingleWarn), or ensure that all items in this list define a value for this metadata.

[Copilot]

@copilot fix this error:

##[error]src/tools/illink/src/ILLink.Tasks/build/Microsoft.NET.ILLink.targets(227,5): error MSB4096: (NETCORE_ENGINEERING_TELEMETRY=Build) The item "/__w/1/s/artifacts/o...

Fixed in 1cb9ce2. The unqualified %(TrimmerSingleWarn) in the Condition was triggering MSBuild to batch over all ResolvedFileToPublish items (not just those being updated), causing MSB4096 for items without that metadata. Changed to %(ResolvedFileToPublish.TrimmerSingleWarn) to qualify it properly.