Fix generic struct delegate in ReadyToRun (#3690)

Fix generic struct delegate in ReadyToRun (#3690)

The following code pattern triggers a crash when compiled into Ready To Run:

    struct S<T>
    {
        public void M() { ... }
    }

    class C { }

    Then create a delegate using S<C>.M

The cause of the crash is calling S<C>.M normally reaches a shared generic
method S<__Canon>.M, but for delegate creation we need to use the actual
S<C>.M. Ready To Run generated code that attempts to create delegate using
S<__Canon>.M, which is not allowed.