Introduce an edge from a const eval to the MIR of all statics it depends on

src/test/incremental/static_refering_to_other_static2/issue.rs

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+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// revisions:rpass1 rpass2
+
+#[cfg(rpass1)]
+pub static A: i32 = 42;
+#[cfg(rpass2)]
+pub static A: i32 = 43;
+
+pub static B: &i32 = &A;
+
+fn main() {}