…in ilc.exe for native aot

- This was causing real C# applications to fail to behave correctly on NativeAOT builds
- Enable testing for covariant byref returns on nativeaot (split testing up so that the tests do not expect TypeLoadException, which NativeAOT doesn't reliably generate)
- Fix implementation of SynthesizedPgoIncompatible project file flag for test script generation
- Put copy of attributetesting.il test into the managed type system unit test suite
- Add regression test of issue noted in dotnet#96175 into managed type system unit test suite
- Update workflow documentation to include a better path to finding details on how to run CoreCLR and Libraries tests for Native AOT

Fixes dotnet#96175

…e for todo comments

…eturns