R2R covariant returns test failures

[BruceForstall]

R2R test failures in apparently every configuration/architecture.

    Loader\classloader\MethodImpl\CovariantReturns\UnitTest\OverrideMoreDerivedReturn\OverrideMoreDerivedReturn.cmd [FAIL]
      
      Assert failure(PID 2908 [0x00000b5c], Thread: 5808 [0x16b0]): HasStableEntryPoint()
      
      CROSSGEN! MethodTable::SetSlot + 0x116 (0x00007ff6`4c19d2ea)
      CROSSGEN! ClassLoader::ValidateMethodsWithCovariantReturnTypes + 0x650 (0x00007ff6`4c1a358c)
      CROSSGEN! PushFinalLevels + 0x23 (0x00007ff6`4c1bba67)
      CROSSGEN! ClassLoader::LoadTypeHandleForTypeKey + 0x197 (0x00007ff6`4c1b9c0f)
      CROSSGEN! ClassLoader::LoadConstructedTypeThrowing + 0x200 (0x00007ff6`4c1b8c84)
      CROSSGEN! ClassLoader::LoadGenericInstantiationThrowing + 0x1ED (0x00007ff6`4c1b8f35)
      CROSSGEN! SigPointer::GetTypeHandleThrowing + 0xB3F (0x00007ff6`4c1c8323)
      CROSSGEN! LoadTypeSpecHelper + 0xC4 (0x00007ff6`4c1ebfd4)
      CROSSGEN! Module::ExpandAll + 0x6D5 (0x00007ff6`4c1e46ed)
      CROSSGEN! CEEPreloader::Preload + 0x7A (0x00007ff6`4c173bfe)
          File: F:\workspace\_work\1\s\src\coreclr\src\vm\method.hpp Line: 248
          Image: C:\h\w\A31108E3\p\crossgen.exe

Starting with: https://dev.azure.com/dnceng/public/_build/results?buildId=797990&view=ms.vss-test-web.build-test-results-tab

Failures include:

    Loader\classloader\MethodImpl\CovariantReturns\UnitTest\OverrideMoreDerivedReturn\OverrideMoreDerivedReturn.cmd [FAIL]
    Loader\classloader\MethodImpl\CovariantReturns\UnitTest\UnitTestDelegates\UnitTestDelegates.cmd [FAIL]
    Loader\classloader\MethodImpl\CovariantReturns\UnitTest\UnitTest_GVM\UnitTest_GVM.cmd [FAIL]
    Loader\classloader\MethodImpl\CovariantReturns\Interfaces\UnitTest\UnitTest.cmd [FAIL]
    Loader\classloader\MethodImpl\CovariantReturns\UnitTest\UnitTest\UnitTest.cmd [FAIL]

Likely candidate PR: #41703

@davidwrighton @janvorli PTAL

[BruceForstall]

cc @mangod9

[janvorli]

@BruceForstall I am already working on a fix.