…derived types of the return type on the MethodDecl.

Feature limitations:
  - Only supports MethodImpls and MethodDecls on classes (no interfaces/valuetypes).
  - MethodImpl and MethodDecl cannot be on the same type.
  - Interface/valuetypes not supported as covariant return types.

Changes are mostly a boolean flag being passed around to allow for covariant type checking in method signatures.
Generics are handled by correctly keeping track of the substitution chain while traversing the base type hierarchy for type comparison.

All method signature comparisons uses metadata for checking, without loading any type (with some exceptions).

Validation for the `ValidateMethodImplRemainsInEffectAttribute` is performed at the very last step of `CLASS_LOAD_EXACTPARENTS`.

Inludes unit tests to cover positive/negative scenarios:
  - Non-generics
  - Generics, with various levels of complexities on generic instantiations and substitutions
  - GVMs
  - Delegates
  - Tests for validation of the existance/absence of the ValidateMethodImplRemainsInEffectAttribute
  - Interface and valuetype scenarios (negative scenarios)

… fails

Fix a couple of issues in IL tests

…ing, and use CanCastTo instead of signature-based checking, to be allow for type compatibility based on ECMA I.8.7.1.

Fixed behavior of the ValidateMethodImplRemainsInEffectAttribute.

Updates to tests

Fixed an assert

…ectAttribute is declared

These changes fix the behavior of the devirtualization algorithm to not incorrectly devirtualize covariant return methods that have the slot unification attribute

Also add tests for those.

Based on PR feedback, I've added test cases to verify the load level
asserts change.

The attempt to overload method with int32 return value by a method with
Nullable<int32> was asserting in CanCastTo. This change fixes it by
pre-checking this case before calling into CanCastTo.
I've also slightly refactored the
ValidateMethodsWithCovariantReturnTypes by extracting the
compatible-with stuff into IsCompatibleWith method.

Added tests to test various compatible-with differences from the
originally used CanCastTo

- Change the CompatibleWithTest to call virtual methods on test
classes instead of just instantiating the classes.
- Update crossgen2 similar to the runtime to have IsCompatibleWith
method doing the same pre-filtering before calling CanCastTo.

The mono doesn't implement the feature yet