Borrows of unsafe immubtable static items is permitted on constructors

[flaper87]

This is currently valid and it shouldn't be.

The issue is in borrowck::gather_loans::mod:: specifically in the gather_loans_for_static_initializer function. This should be fixed by implementing a small Visitor that will walk the passed expr and look for cases like the one in the example below:

struct Wrap<T> { value: T }
static UNSAFE: Unsafe<int> = Unsafe{value: 1, marker1: marker::InvariantType};
static WRAPPED_UNSAFE: Wrap<&'static Unsafe<int>> = Wrap { value: &UNSAFE };

fn main() {}

cc @nikomatsakis