C#: Use static constructor as enclosing callable for static member initializers.

[michaelnebel]

It appears that the cs/useless-assignment-to-local produces some false positives for static members.
That is, the query incorrectly reports x as being a useless assignment in

public static readonly IEnumerable<int> StaticQueryBackedField =
        from x in new[] { 1, 2, 3 }
        where x > 1
        select x;

The root cause of the problem is that the static member initializers don't have an enclosing callable (which affects the SSA logic and this bubbles up into the results of cs/useless-assignment-to-local query). In this PR we fix the problem by using the static constructor as the enclosing callable for the control flow elements/expressions of static member inititializers. For instance fields, there is a dedicated synthetic "object initializer" method (as there can exist multiple instance constructors). However, there is exactly one static constructor, so we can use that as the enclosing callable.