Fix covariant returns when overriding method of non-parent ancestor

[janvorli]

There is a problem in the
ClassLoader::ValidateMethodsWithCovariantReturnTypes that results in
failed verification of valid override in case the return type of the
method being overriden is generic in canonical form and it is defined
in an ancestor class that is not the parent.

The problem is that we attempt to use instantiation of the parent class
instead of the ancestor class that contains definition of the method
being overridden.

This change fixes it by locating the proper ancestor MethodTable and
using it.

Close #45053

[janvorli]

/backport to release/5.0

[github-actions]

Started backporting to release/5.0: https://github.com/dotnet/runtime/actions/runs/539974060

[github-actions]

@janvorli backporting to release/5.0 failed, the patch most likely resulted in conflicts:

$ git am --3way --ignore-whitespace --keep-non-patch changes.patch

Applying: Fix covariant returns when overriding method of non-parent ancestor
error: sha1 information is lacking or useless (src/coreclr/vm/class.cpp).
error: could not build fake ancestor
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 Fix covariant returns when overriding method of non-parent ancestor
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".
Error: The process '/usr/bin/git' failed with exit code 128

Please backport manually!