I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

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This also needs:

• To add these files to the csproj: https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
• XML comments on the APIs to seed the documentation
• To add these to the refs: https://github.com/dotnet/runtime/blob/main/src/libraries/System.Runtime/ref/System.Runtime.cs
• Minimal tests, ala https://github.com/dotnet/runtime/blob/main/src/libraries/System.Runtime/tests/System/Diagnostics/CodeAnalysis/ConstantExpectedAttributeTests.cs. Given there are no members, the test will just validate the instance can be constructed.

Also the ifdef for corelib is only necessary if we expect these files to be included into other assemblies beyond corelib.

Also the ifdef for corelib is only necessary if we expect these files to be included into other assemblies beyond corelib.

I somewhat expect that this attribute will be copied by customers, so I'd prefer to keep them.