Add code for linker to generate xml warning suppressions when builds fail

[layomia]

Makes it a bit easier to suppress linker warnings in PRs such as https://github.com/dotnet/runtime/pull/43965/checks?check_run_id=1355002347. With this change the following steps can be taken:

1. Build locally using the command failing in CI, adding /p:GenerateLinkerWarningSuppressions=true,
   • e.g. build.cmd -subset libs -configuration Debug -ci -allConfigurations /p:GenerateLinkerWarningSuppressions=true from this failure log.
2. Find suppression XML file(s) generated to the local/config-dependent equivalent of this location: runtime\artifacts\bin\ILLinkTrimAssembly\net6.0-windows-Release-x64\trimmed-runtimepack
3. Move the contents of the generated XML files to the corresponding XML linker suppression file location for the erring assembly (or create one if necessary). e.g, suppression data for warnings in System.Net.Security would go to a file named runtime\src\libraries\System.Net.Security\src\ILLink\ILLink.Suppressions.xml.
   • Note that there might need to be a build-configuration specific file, e.g. ILLink.Suppressions.Windows_NT.xml. See this code for some existing conventions. A new convention might need to be added, depending on the failing scenario. Also see Add granular suppressions for linker warnings #40691 which added several XML suppression files to address Enable linker analysis and add baselines for warnings #38033.

TODO:

• Document these steps somewhere appropriate
• Investigate a way to automatically make such changes (e.g. an MSBuild target). We should be sure not to lower the bar for adding new linker-unfriendly code as a result.

[ghost]

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[layomia]

FWIW - At the moment, I'm unsure why #43965 causes new linker warnings but the steps above should help resolve them.

[ericstj]

@layomia @eerhardt is this still needed?

[layomia]

@ericstj - yes, it just needs some doc additions. Will circle back to this.

[ericstj]

I'm going to go ahead and close this one-line PR. Please feel free to open a new PR when you're ready to resume this work if it's still necessary.