ability for global variables to be initialized with address of each other

[andrewrk]

const std = @import("std");
const expect = std.testing.expect;

const Node = struct {
 next: *Node,
};

const a: Node = .{ .next = &b };
const b: Node = .{ .next = &a };

test "example" {
 expect(a.next == &b);
 expect(b.next == &a);
}

Currently, this test case produces the following results:

test.zig:8:1: error: dependency loop detected
const a: Node = .{ .next = &b };
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
referenced by:
 a: test.zig:8:29
 b: test.zig:9:29
 remaining reference traces hidden; use '-freference-trace' to see all reference traces