[corlib] Fix regression in Delegate::Equal found by gtest-etree-01.
[runtime] When parsing a method header, ensure we pass any available generic context. Fixes #21388.

We need to pass the right generic context to the method parsing so local variables with
generic argument types get the owner properly set.

In addition to that, harden the icall in case the owner is not set.
[corlib] Add regression test for #21388.